[PATCH] zswap: only save zswap header if zpool is shrinkable

[PATCH] zswap: only save zswap header if zpool is shrinkable

[Yu Zhao]

We waste sizeof(swp_entry_t) for zswap header when using zsmalloc
as zpool driver because zsmalloc doesn't support eviction.

Add zpool_shrinkable() to detect if zpool is shrinkable, and use
it in zswap to avoid waste memory for zswap header.

Signed-off-by: Yu Zhao <yuzhao@google.com>
---
 include/linux/zpool.h |  2 ++
 mm/zpool.c            | 17 ++++++++++++++++-
 mm/zsmalloc.c         |  7 -------
 mm/zswap.c            | 20 ++++++++++----------
 4 files changed, 28 insertions(+), 18 deletions(-)

diff --git a/include/linux/zpool.h b/include/linux/zpool.h
index 004ba807df96..3f0ac2ab74aa 100644
--- a/include/linux/zpool.h
+++ b/include/linux/zpool.h
@@ -108,4 +108,6 @@ void zpool_register_driver(struct zpool_driver *driver);
 
 int zpool_unregister_driver(struct zpool_driver *driver);
 
+bool zpool_shrinkable(struct zpool *pool);
+
 #endif
diff --git a/mm/zpool.c b/mm/zpool.c
index fd3ff719c32c..839d4234c540 100644
--- a/mm/zpool.c
+++ b/mm/zpool.c
@@ -296,7 +296,8 @@ void zpool_free(struct zpool *zpool, unsigned long handle)
 int zpool_shrink(struct zpool *zpool, unsigned int pages,
 			unsigned int *reclaimed)
 {
-	return zpool->driver->shrink(zpool->pool, pages, reclaimed);
+	return zpool_shrinkable(zpool) ?
+	       zpool->driver->shrink(zpool->pool, pages, reclaimed) : -EINVAL;
 }
 
 /**
@@ -355,6 +356,20 @@ u64 zpool_get_total_size(struct zpool *zpool)
 	return zpool->driver->total_size(zpool->pool);
 }
 
+/**
+ * zpool_shrinkable() - Test if zpool is shrinkable
+ * @pool	The zpool to test
+ *
+ * Zpool is only shrinkable when it's created with struct
+ * zpool_ops.evict and its driver implements struct zpool_driver.shrink.
+ *
+ * Returns: true if shrinkable; false otherwise.
+ */
+bool zpool_shrinkable(struct zpool *zpool)
+{
+	return zpool->ops && zpool->ops->evict && zpool->driver->shrink;
+}
+
 MODULE_LICENSE("GPL");
 MODULE_AUTHOR("Dan Streetman <ddstreet@ieee.org>");
 MODULE_DESCRIPTION("Common API for compressed memory storage");
diff --git a/mm/zsmalloc.c b/mm/zsmalloc.c
index 683c0651098c..9cc741bcdb32 100644
--- a/mm/zsmalloc.c
+++ b/mm/zsmalloc.c
@@ -407,12 +407,6 @@ static void zs_zpool_free(void *pool, unsigned long handle)
 	zs_free(pool, handle);
 }
 
-static int zs_zpool_shrink(void *pool, unsigned int pages,
-			unsigned int *reclaimed)
-{
-	return -EINVAL;
-}
-
 static void *zs_zpool_map(void *pool, unsigned long handle,
 			enum zpool_mapmode mm)
 {
@@ -450,7 +444,6 @@ static struct zpool_driver zs_zpool_driver = {
 	.destroy =	zs_zpool_destroy,
 	.malloc =	zs_zpool_malloc,
 	.free =		zs_zpool_free,
-	.shrink =	zs_zpool_shrink,
 	.map =		zs_zpool_map,
 	.unmap =	zs_zpool_unmap,
 	.total_size =	zs_zpool_total_size,
diff --git a/mm/zswap.c b/mm/zswap.c
index d39581a076c3..15d2ea29a6fa 100644
--- a/mm/zswap.c
+++ b/mm/zswap.c
@@ -964,11 +964,11 @@ static int zswap_frontswap_store(unsigned type, pgoff_t offset,
 	struct zswap_entry *entry, *dupentry;
 	struct crypto_comp *tfm;
 	int ret;
-	unsigned int dlen = PAGE_SIZE, len;
+	unsigned int hlen, dlen = PAGE_SIZE;
 	unsigned long handle;
 	char *buf;
 	u8 *src, *dst;
-	struct zswap_header *zhdr;
+	struct zswap_header zhdr = { .swpentry = swp_entry(type, offset) };
 
 	if (!zswap_enabled || !tree) {
 		ret = -ENODEV;
@@ -1013,8 +1013,8 @@ static int zswap_frontswap_store(unsigned type, pgoff_t offset,
 	}
 
 	/* store */
-	len = dlen + sizeof(struct zswap_header);
-	ret = zpool_malloc(entry->pool->zpool, len,
+	hlen = zpool_shrinkable(entry->pool->zpool) ? sizeof(zhdr) : 0;
+	ret = zpool_malloc(entry->pool->zpool, hlen + dlen,
 			   __GFP_NORETRY | __GFP_NOWARN | __GFP_KSWAPD_RECLAIM,
 			   &handle);
 	if (ret == -ENOSPC) {
@@ -1025,10 +1025,9 @@ static int zswap_frontswap_store(unsigned type, pgoff_t offset,
 		zswap_reject_alloc_fail++;
 		goto put_dstmem;
 	}
-	zhdr = zpool_map_handle(entry->pool->zpool, handle, ZPOOL_MM_RW);
-	zhdr->swpentry = swp_entry(type, offset);
-	buf = (u8 *)(zhdr + 1);
-	memcpy(buf, dst, dlen);
+	buf = zpool_map_handle(entry->pool->zpool, handle, ZPOOL_MM_RW);
+	memcpy(buf, &zhdr, hlen);
+	memcpy(buf + hlen, dst, dlen);
 	zpool_unmap_handle(entry->pool->zpool, handle);
 	put_cpu_var(zswap_dstmem);
 
@@ -1091,8 +1090,9 @@ static int zswap_frontswap_load(unsigned type, pgoff_t offset,
 
 	/* decompress */
 	dlen = PAGE_SIZE;
-	src = (u8 *)zpool_map_handle(entry->pool->zpool, entry->handle,
-			ZPOOL_MM_RO) + sizeof(struct zswap_header);
+	src = zpool_map_handle(entry->pool->zpool, entry->handle, ZPOOL_MM_RO);
+	if (zpool_shrinkable(entry->pool->zpool))
+		src += sizeof(struct zswap_header);
 	dst = kmap_atomic(page);
 	tfm = *get_cpu_ptr(entry->pool->tfm);
 	ret = crypto_comp_decompress(tfm, src, entry->length, dst, &dlen);
-- 
2.16.0.rc0.223.g4a4ac83678-goog

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Re: [PATCH] zswap: only save zswap header if zpool is shrinkable

[Sergey Senozhatsky]

On (01/08/18 14:51), Yu Zhao wrote:
> We waste sizeof(swp_entry_t) for zswap header when using zsmalloc
> as zpool driver because zsmalloc doesn't support eviction.
> 
> Add zpool_shrinkable() to detect if zpool is shrinkable, and use
> it in zswap to avoid waste memory for zswap header.
> 
> Signed-off-by: Yu Zhao <yuzhao@google.com>

at a glance, looks good to me
Reviewed-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>

	-ss

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Re: [PATCH] zswap: only save zswap header if zpool is shrinkable

[Sergey Senozhatsky]

On (01/08/18 14:51), Yu Zhao wrote:
[..]
>  int zpool_shrink(struct zpool *zpool, unsigned int pages,
>  			unsigned int *reclaimed)
>  {
> -	return zpool->driver->shrink(zpool->pool, pages, reclaimed);
> +	return zpool_shrinkable(zpool) ?
> +	       zpool->driver->shrink(zpool->pool, pages, reclaimed) : -EINVAL;
>  }
>  
>  /**
> @@ -355,6 +356,20 @@ u64 zpool_get_total_size(struct zpool *zpool)
>  	return zpool->driver->total_size(zpool->pool);
>  }
>  
> +/**
> + * zpool_shrinkable() - Test if zpool is shrinkable
> + * @pool	The zpool to test
> + *
> + * Zpool is only shrinkable when it's created with struct
> + * zpool_ops.evict and its driver implements struct zpool_driver.shrink.
> + *
> + * Returns: true if shrinkable; false otherwise.
> + */
> +bool zpool_shrinkable(struct zpool *zpool)
> +{
> +	return zpool->ops && zpool->ops->evict && zpool->driver->shrink;
> +}

just a side note,
it might be a bit confusing and maybe there is a better
name for it. zsmalloc is shrinkable (we register a shrinker
callback), but not in the way zpool defines it.

	-ss

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Re: [PATCH] zswap: only save zswap header if zpool is shrinkable

[Yu Zhao]

On Tue, Jan 09, 2018 at 01:48:17PM +0900, Sergey Senozhatsky wrote:
> On (01/08/18 14:51), Yu Zhao wrote:
> [..]
> >  int zpool_shrink(struct zpool *zpool, unsigned int pages,
> >  			unsigned int *reclaimed)
> >  {
> > -	return zpool->driver->shrink(zpool->pool, pages, reclaimed);
> > +	return zpool_shrinkable(zpool) ?
> > +	       zpool->driver->shrink(zpool->pool, pages, reclaimed) : -EINVAL;
> >  }
> >  
> >  /**
> > @@ -355,6 +356,20 @@ u64 zpool_get_total_size(struct zpool *zpool)
> >  	return zpool->driver->total_size(zpool->pool);
> >  }
> >  
> > +/**
> > + * zpool_shrinkable() - Test if zpool is shrinkable
> > + * @pool	The zpool to test
> > + *
> > + * Zpool is only shrinkable when it's created with struct
> > + * zpool_ops.evict and its driver implements struct zpool_driver.shrink.
> > + *
> > + * Returns: true if shrinkable; false otherwise.
> > + */
> > +bool zpool_shrinkable(struct zpool *zpool)
> > +{
> > +	return zpool->ops && zpool->ops->evict && zpool->driver->shrink;
> > +}
> 
> just a side note,
> it might be a bit confusing and maybe there is a better
> name for it. zsmalloc is shrinkable (we register a shrinker
> callback), but not in the way zpool defines it.

Thanks. Do zpool_evictable() and zpool->driver->evict make more sense?

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Re: [PATCH] zswap: only save zswap header if zpool is shrinkable

[Dan Streetman]

On Mon, Jan 8, 2018 at 5:51 PM, Yu Zhao <yuzhao@google.com> wrote:
> We waste sizeof(swp_entry_t) for zswap header when using zsmalloc
> as zpool driver because zsmalloc doesn't support eviction.
>
> Add zpool_shrinkable() to detect if zpool is shrinkable, and use
> it in zswap to avoid waste memory for zswap header.
>
> Signed-off-by: Yu Zhao <yuzhao@google.com>
> ---
>  include/linux/zpool.h |  2 ++
>  mm/zpool.c            | 17 ++++++++++++++++-
>  mm/zsmalloc.c         |  7 -------
>  mm/zswap.c            | 20 ++++++++++----------
>  4 files changed, 28 insertions(+), 18 deletions(-)
>
> diff --git a/include/linux/zpool.h b/include/linux/zpool.h
> index 004ba807df96..3f0ac2ab74aa 100644
> --- a/include/linux/zpool.h
> +++ b/include/linux/zpool.h
> @@ -108,4 +108,6 @@ void zpool_register_driver(struct zpool_driver *driver);
>
>  int zpool_unregister_driver(struct zpool_driver *driver);
>
> +bool zpool_shrinkable(struct zpool *pool);
> +
>  #endif
> diff --git a/mm/zpool.c b/mm/zpool.c
> index fd3ff719c32c..839d4234c540 100644
> --- a/mm/zpool.c
> +++ b/mm/zpool.c
> @@ -296,7 +296,8 @@ void zpool_free(struct zpool *zpool, unsigned long handle)
>  int zpool_shrink(struct zpool *zpool, unsigned int pages,
>                         unsigned int *reclaimed)
>  {
> -       return zpool->driver->shrink(zpool->pool, pages, reclaimed);
> +       return zpool_shrinkable(zpool) ?
> +              zpool->driver->shrink(zpool->pool, pages, reclaimed) : -EINVAL;
>  }
>
>  /**
> @@ -355,6 +356,20 @@ u64 zpool_get_total_size(struct zpool *zpool)
>         return zpool->driver->total_size(zpool->pool);
>  }
>
> +/**
> + * zpool_shrinkable() - Test if zpool is shrinkable
> + * @pool       The zpool to test
> + *
> + * Zpool is only shrinkable when it's created with struct
> + * zpool_ops.evict and its driver implements struct zpool_driver.shrink.
> + *
> + * Returns: true if shrinkable; false otherwise.
> + */
> +bool zpool_shrinkable(struct zpool *zpool)
> +{
> +       return zpool->ops && zpool->ops->evict && zpool->driver->shrink;

as these things won't ever change for the life of the zpool, it would
probably be better to just check them at zpool creation time and set a
single new zpool param, like 'zpool->shrinkable'. since this function
will be called for every page that's swapped in or out, that may save
a bit of time.

also re: calling it 'shrinkable' or 'evictable', the real thing zswap
is interested in is if it needs to include the header info that
zswap_writeback_entry (i.e. ops->evict) later needs, so yeah it does
make more sense to call it zpool_evictable() and zpool->evictable.
However, I think the function should still be zpool_shrink() and
zpool->driver->shrink(), because it should be possible for
zs_pool_shrink() to call the normal zsmalloc shrinker, instead of
doing the zswap-style eviction, even if it doesn't do that currently.

> +}
> +
>  MODULE_LICENSE("GPL");
>  MODULE_AUTHOR("Dan Streetman <ddstreet@ieee.org>");
>  MODULE_DESCRIPTION("Common API for compressed memory storage");
> diff --git a/mm/zsmalloc.c b/mm/zsmalloc.c
> index 683c0651098c..9cc741bcdb32 100644
> --- a/mm/zsmalloc.c
> +++ b/mm/zsmalloc.c
> @@ -407,12 +407,6 @@ static void zs_zpool_free(void *pool, unsigned long handle)
>         zs_free(pool, handle);
>  }
>
> -static int zs_zpool_shrink(void *pool, unsigned int pages,
> -                       unsigned int *reclaimed)
> -{
> -       return -EINVAL;
> -}
> -
>  static void *zs_zpool_map(void *pool, unsigned long handle,
>                         enum zpool_mapmode mm)
>  {
> @@ -450,7 +444,6 @@ static struct zpool_driver zs_zpool_driver = {
>         .destroy =      zs_zpool_destroy,
>         .malloc =       zs_zpool_malloc,
>         .free =         zs_zpool_free,
> -       .shrink =       zs_zpool_shrink,
>         .map =          zs_zpool_map,
>         .unmap =        zs_zpool_unmap,
>         .total_size =   zs_zpool_total_size,
> diff --git a/mm/zswap.c b/mm/zswap.c
> index d39581a076c3..15d2ea29a6fa 100644
> --- a/mm/zswap.c
> +++ b/mm/zswap.c
> @@ -964,11 +964,11 @@ static int zswap_frontswap_store(unsigned type, pgoff_t offset,
>         struct zswap_entry *entry, *dupentry;
>         struct crypto_comp *tfm;
>         int ret;
> -       unsigned int dlen = PAGE_SIZE, len;
> +       unsigned int hlen, dlen = PAGE_SIZE;
>         unsigned long handle;
>         char *buf;
>         u8 *src, *dst;
> -       struct zswap_header *zhdr;
> +       struct zswap_header zhdr = { .swpentry = swp_entry(type, offset) };
>
>         if (!zswap_enabled || !tree) {
>                 ret = -ENODEV;
> @@ -1013,8 +1013,8 @@ static int zswap_frontswap_store(unsigned type, pgoff_t offset,
>         }
>
>         /* store */
> -       len = dlen + sizeof(struct zswap_header);
> -       ret = zpool_malloc(entry->pool->zpool, len,
> +       hlen = zpool_shrinkable(entry->pool->zpool) ? sizeof(zhdr) : 0;
> +       ret = zpool_malloc(entry->pool->zpool, hlen + dlen,
>                            __GFP_NORETRY | __GFP_NOWARN | __GFP_KSWAPD_RECLAIM,
>                            &handle);
>         if (ret == -ENOSPC) {
> @@ -1025,10 +1025,9 @@ static int zswap_frontswap_store(unsigned type, pgoff_t offset,
>                 zswap_reject_alloc_fail++;
>                 goto put_dstmem;
>         }
> -       zhdr = zpool_map_handle(entry->pool->zpool, handle, ZPOOL_MM_RW);
> -       zhdr->swpentry = swp_entry(type, offset);
> -       buf = (u8 *)(zhdr + 1);
> -       memcpy(buf, dst, dlen);
> +       buf = zpool_map_handle(entry->pool->zpool, handle, ZPOOL_MM_RW);
> +       memcpy(buf, &zhdr, hlen);
> +       memcpy(buf + hlen, dst, dlen);
>         zpool_unmap_handle(entry->pool->zpool, handle);
>         put_cpu_var(zswap_dstmem);
>
> @@ -1091,8 +1090,9 @@ static int zswap_frontswap_load(unsigned type, pgoff_t offset,
>
>         /* decompress */
>         dlen = PAGE_SIZE;
> -       src = (u8 *)zpool_map_handle(entry->pool->zpool, entry->handle,
> -                       ZPOOL_MM_RO) + sizeof(struct zswap_header);
> +       src = zpool_map_handle(entry->pool->zpool, entry->handle, ZPOOL_MM_RO);
> +       if (zpool_shrinkable(entry->pool->zpool))
> +               src += sizeof(struct zswap_header);
>         dst = kmap_atomic(page);
>         tfm = *get_cpu_ptr(entry->pool->tfm);
>         ret = crypto_comp_decompress(tfm, src, entry->length, dst, &dlen);
> --
> 2.16.0.rc0.223.g4a4ac83678-goog
>

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Re: [PATCH] zswap: only save zswap header if zpool is shrinkable

[Yu Zhao]

On Tue, Jan 09, 2018 at 01:25:18PM -0500, Dan Streetman wrote:
> On Mon, Jan 8, 2018 at 5:51 PM, Yu Zhao <yuzhao@google.com> wrote:
> > We waste sizeof(swp_entry_t) for zswap header when using zsmalloc
> > as zpool driver because zsmalloc doesn't support eviction.
> >
> > Add zpool_shrinkable() to detect if zpool is shrinkable, and use
> > it in zswap to avoid waste memory for zswap header.
> >
> > Signed-off-by: Yu Zhao <yuzhao@google.com>
> > ---
> >  include/linux/zpool.h |  2 ++
> >  mm/zpool.c            | 17 ++++++++++++++++-
> >  mm/zsmalloc.c         |  7 -------
> >  mm/zswap.c            | 20 ++++++++++----------
> >  4 files changed, 28 insertions(+), 18 deletions(-)
> >
> > diff --git a/include/linux/zpool.h b/include/linux/zpool.h
> > index 004ba807df96..3f0ac2ab74aa 100644
> > --- a/include/linux/zpool.h
> > +++ b/include/linux/zpool.h
> > @@ -108,4 +108,6 @@ void zpool_register_driver(struct zpool_driver *driver);
> >
> >  int zpool_unregister_driver(struct zpool_driver *driver);
> >
> > +bool zpool_shrinkable(struct zpool *pool);
> > +
> >  #endif
> > diff --git a/mm/zpool.c b/mm/zpool.c
> > index fd3ff719c32c..839d4234c540 100644
> > --- a/mm/zpool.c
> > +++ b/mm/zpool.c
> > @@ -296,7 +296,8 @@ void zpool_free(struct zpool *zpool, unsigned long handle)
> >  int zpool_shrink(struct zpool *zpool, unsigned int pages,
> >                         unsigned int *reclaimed)
> >  {
> > -       return zpool->driver->shrink(zpool->pool, pages, reclaimed);
> > +       return zpool_shrinkable(zpool) ?
> > +              zpool->driver->shrink(zpool->pool, pages, reclaimed) : -EINVAL;
> >  }
> >
> >  /**
> > @@ -355,6 +356,20 @@ u64 zpool_get_total_size(struct zpool *zpool)
> >         return zpool->driver->total_size(zpool->pool);
> >  }
> >
> > +/**
> > + * zpool_shrinkable() - Test if zpool is shrinkable
> > + * @pool       The zpool to test
> > + *
> > + * Zpool is only shrinkable when it's created with struct
> > + * zpool_ops.evict and its driver implements struct zpool_driver.shrink.
> > + *
> > + * Returns: true if shrinkable; false otherwise.
> > + */
> > +bool zpool_shrinkable(struct zpool *zpool)
> > +{
> > +       return zpool->ops && zpool->ops->evict && zpool->driver->shrink;
> 
> as these things won't ever change for the life of the zpool, it would
> probably be better to just check them at zpool creation time and set a
> single new zpool param, like 'zpool->shrinkable'. since this function
> will be called for every page that's swapped in or out, that may save
> a bit of time.

Ack.

> also re: calling it 'shrinkable' or 'evictable', the real thing zswap
> is interested in is if it needs to include the header info that
> zswap_writeback_entry (i.e. ops->evict) later needs, so yeah it does
> make more sense to call it zpool_evictable() and zpool->evictable.
> However, I think the function should still be zpool_shrink() and
> zpool->driver->shrink(), because it should be possible for
> zs_pool_shrink() to call the normal zsmalloc shrinker, instead of
> doing the zswap-style eviction, even if it doesn't do that currently.

I agree we keep zpool_shrink(). It could either shrink pool if driver
supports slab shrinker by providing zpool->driver->shrink or evict
pages from pool if driver supports zpool->driver->evict (which in turn
calls ops->evict provided by zswap) or both.

We can't use a single zpool->driver->callback to achieve both because
there will be no way for zswap to know if driver uses ops->evict thus
no way to determine if zswap_header is needed.

So for now, I think it'd be better if we deleted zpool->driver->shrink
from zsmalloc and renamed it to zpool->driver->evict in zbud. Later
if we decide zpool_shrink should also call zsmalloc slab shrinker, we
add a new callback.

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Re: [PATCH] zswap: only save zswap header if zpool is shrinkable

[Dan Streetman]

On Tue, Jan 9, 2018 at 5:47 PM, Yu Zhao <yuzhao@google.com> wrote:
> On Tue, Jan 09, 2018 at 01:25:18PM -0500, Dan Streetman wrote:
>> On Mon, Jan 8, 2018 at 5:51 PM, Yu Zhao <yuzhao@google.com> wrote:
>> > We waste sizeof(swp_entry_t) for zswap header when using zsmalloc
>> > as zpool driver because zsmalloc doesn't support eviction.
>> >
>> > Add zpool_shrinkable() to detect if zpool is shrinkable, and use
>> > it in zswap to avoid waste memory for zswap header.
>> >
>> > Signed-off-by: Yu Zhao <yuzhao@google.com>
>> > ---
>> >  include/linux/zpool.h |  2 ++
>> >  mm/zpool.c            | 17 ++++++++++++++++-
>> >  mm/zsmalloc.c         |  7 -------
>> >  mm/zswap.c            | 20 ++++++++++----------
>> >  4 files changed, 28 insertions(+), 18 deletions(-)
>> >
>> > diff --git a/include/linux/zpool.h b/include/linux/zpool.h
>> > index 004ba807df96..3f0ac2ab74aa 100644
>> > --- a/include/linux/zpool.h
>> > +++ b/include/linux/zpool.h
>> > @@ -108,4 +108,6 @@ void zpool_register_driver(struct zpool_driver *driver);
>> >
>> >  int zpool_unregister_driver(struct zpool_driver *driver);
>> >
>> > +bool zpool_shrinkable(struct zpool *pool);
>> > +
>> >  #endif
>> > diff --git a/mm/zpool.c b/mm/zpool.c
>> > index fd3ff719c32c..839d4234c540 100644
>> > --- a/mm/zpool.c
>> > +++ b/mm/zpool.c
>> > @@ -296,7 +296,8 @@ void zpool_free(struct zpool *zpool, unsigned long handle)
>> >  int zpool_shrink(struct zpool *zpool, unsigned int pages,
>> >                         unsigned int *reclaimed)
>> >  {
>> > -       return zpool->driver->shrink(zpool->pool, pages, reclaimed);
>> > +       return zpool_shrinkable(zpool) ?
>> > +              zpool->driver->shrink(zpool->pool, pages, reclaimed) : -EINVAL;
>> >  }
>> >
>> >  /**
>> > @@ -355,6 +356,20 @@ u64 zpool_get_total_size(struct zpool *zpool)
>> >         return zpool->driver->total_size(zpool->pool);
>> >  }
>> >
>> > +/**
>> > + * zpool_shrinkable() - Test if zpool is shrinkable
>> > + * @pool       The zpool to test
>> > + *
>> > + * Zpool is only shrinkable when it's created with struct
>> > + * zpool_ops.evict and its driver implements struct zpool_driver.shrink.
>> > + *
>> > + * Returns: true if shrinkable; false otherwise.
>> > + */
>> > +bool zpool_shrinkable(struct zpool *zpool)
>> > +{
>> > +       return zpool->ops && zpool->ops->evict && zpool->driver->shrink;
>>
>> as these things won't ever change for the life of the zpool, it would
>> probably be better to just check them at zpool creation time and set a
>> single new zpool param, like 'zpool->shrinkable'. since this function
>> will be called for every page that's swapped in or out, that may save
>> a bit of time.
>
> Ack.
>
>> also re: calling it 'shrinkable' or 'evictable', the real thing zswap
>> is interested in is if it needs to include the header info that
>> zswap_writeback_entry (i.e. ops->evict) later needs, so yeah it does
>> make more sense to call it zpool_evictable() and zpool->evictable.
>> However, I think the function should still be zpool_shrink() and
>> zpool->driver->shrink(), because it should be possible for
>> zs_pool_shrink() to call the normal zsmalloc shrinker, instead of
>> doing the zswap-style eviction, even if it doesn't do that currently.
>
> I agree we keep zpool_shrink(). It could either shrink pool if driver
> supports slab shrinker by providing zpool->driver->shrink or evict
> pages from pool if driver supports zpool->driver->evict (which in turn
> calls ops->evict provided by zswap) or both.
>
> We can't use a single zpool->driver->callback to achieve both because
> there will be no way for zswap to know if driver uses ops->evict thus
> no way to determine if zswap_header is needed.
>
> So for now, I think it'd be better if we deleted zpool->driver->shrink
> from zsmalloc and renamed it to zpool->driver->evict in zbud. Later
> if we decide zpool_shrink should also call zsmalloc slab shrinker, we
> add a new callback.

Well, I think shrink vs evict an implementation detail, isn't it?
That is, from zswap's perspective, there should be:

zpool_evictable()
if true, zswap needs to include the header on each compressed page,
because the zpool may callback zpool->ops->evict() which calls
zswap_writeback_entry() which expects the entry to start with a zswap
header.
if false, zswap doesn't need to include the header, because the zpool
will never, ever call zpool->ops->evict

zpool_shrink()
this will try to shrink the zpool, using whatever
zpool-implementation-specific shrinking method.  If zpool_evictable()
is true for this zpool, then zpool_shrink() *might* callback to
zpool->ops->evict(), although it doesn't have to if it can shrink
without evictions.  If zpool_evictable() is false, then zpool_shrink()
will never callback to zpool->ops->evict().

There is really no need for zswap to call different functions based on
whether the pool is evictable or not...is there?

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Re: [PATCH] zswap: only save zswap header if zpool is shrinkable

[Yu Zhao]

On Wed, Jan 10, 2018 at 03:06:47PM -0500, Dan Streetman wrote:
> On Tue, Jan 9, 2018 at 5:47 PM, Yu Zhao <yuzhao@google.com> wrote:
> > On Tue, Jan 09, 2018 at 01:25:18PM -0500, Dan Streetman wrote:
> >> On Mon, Jan 8, 2018 at 5:51 PM, Yu Zhao <yuzhao@google.com> wrote:
> >> > We waste sizeof(swp_entry_t) for zswap header when using zsmalloc
> >> > as zpool driver because zsmalloc doesn't support eviction.
> >> >
> >> > Add zpool_shrinkable() to detect if zpool is shrinkable, and use
> >> > it in zswap to avoid waste memory for zswap header.
> >> >
> >> > Signed-off-by: Yu Zhao <yuzhao@google.com>
> >> > ---
> >> >  include/linux/zpool.h |  2 ++
> >> >  mm/zpool.c            | 17 ++++++++++++++++-
> >> >  mm/zsmalloc.c         |  7 -------
> >> >  mm/zswap.c            | 20 ++++++++++----------
> >> >  4 files changed, 28 insertions(+), 18 deletions(-)
> >> >
> >> > diff --git a/include/linux/zpool.h b/include/linux/zpool.h
> >> > index 004ba807df96..3f0ac2ab74aa 100644
> >> > --- a/include/linux/zpool.h
> >> > +++ b/include/linux/zpool.h
> >> > @@ -108,4 +108,6 @@ void zpool_register_driver(struct zpool_driver *driver);
> >> >
> >> >  int zpool_unregister_driver(struct zpool_driver *driver);
> >> >
> >> > +bool zpool_shrinkable(struct zpool *pool);
> >> > +
> >> >  #endif
> >> > diff --git a/mm/zpool.c b/mm/zpool.c
> >> > index fd3ff719c32c..839d4234c540 100644
> >> > --- a/mm/zpool.c
> >> > +++ b/mm/zpool.c
> >> > @@ -296,7 +296,8 @@ void zpool_free(struct zpool *zpool, unsigned long handle)
> >> >  int zpool_shrink(struct zpool *zpool, unsigned int pages,
> >> >                         unsigned int *reclaimed)
> >> >  {
> >> > -       return zpool->driver->shrink(zpool->pool, pages, reclaimed);
> >> > +       return zpool_shrinkable(zpool) ?
> >> > +              zpool->driver->shrink(zpool->pool, pages, reclaimed) : -EINVAL;
> >> >  }
> >> >
> >> >  /**
> >> > @@ -355,6 +356,20 @@ u64 zpool_get_total_size(struct zpool *zpool)
> >> >         return zpool->driver->total_size(zpool->pool);
> >> >  }
> >> >
> >> > +/**
> >> > + * zpool_shrinkable() - Test if zpool is shrinkable
> >> > + * @pool       The zpool to test
> >> > + *
> >> > + * Zpool is only shrinkable when it's created with struct
> >> > + * zpool_ops.evict and its driver implements struct zpool_driver.shrink.
> >> > + *
> >> > + * Returns: true if shrinkable; false otherwise.
> >> > + */
> >> > +bool zpool_shrinkable(struct zpool *zpool)
> >> > +{
> >> > +       return zpool->ops && zpool->ops->evict && zpool->driver->shrink;
> >>
> >> as these things won't ever change for the life of the zpool, it would
> >> probably be better to just check them at zpool creation time and set a
> >> single new zpool param, like 'zpool->shrinkable'. since this function
> >> will be called for every page that's swapped in or out, that may save
> >> a bit of time.
> >
> > Ack.
> >
> >> also re: calling it 'shrinkable' or 'evictable', the real thing zswap
> >> is interested in is if it needs to include the header info that
> >> zswap_writeback_entry (i.e. ops->evict) later needs, so yeah it does
> >> make more sense to call it zpool_evictable() and zpool->evictable.
> >> However, I think the function should still be zpool_shrink() and
> >> zpool->driver->shrink(), because it should be possible for
> >> zs_pool_shrink() to call the normal zsmalloc shrinker, instead of
> >> doing the zswap-style eviction, even if it doesn't do that currently.
> >
> > I agree we keep zpool_shrink(). It could either shrink pool if driver
> > supports slab shrinker by providing zpool->driver->shrink or evict
> > pages from pool if driver supports zpool->driver->evict (which in turn
> > calls ops->evict provided by zswap) or both.
> >
> > We can't use a single zpool->driver->callback to achieve both because
> > there will be no way for zswap to know if driver uses ops->evict thus
> > no way to determine if zswap_header is needed.
> >
> > So for now, I think it'd be better if we deleted zpool->driver->shrink
> > from zsmalloc and renamed it to zpool->driver->evict in zbud. Later
> > if we decide zpool_shrink should also call zsmalloc slab shrinker, we
> > add a new callback.
> 
> Well, I think shrink vs evict an implementation detail, isn't it?
> That is, from zswap's perspective, there should be:
> 
> zpool_evictable()
> if true, zswap needs to include the header on each compressed page,
> because the zpool may callback zpool->ops->evict() which calls
> zswap_writeback_entry() which expects the entry to start with a zswap
> header.
> if false, zswap doesn't need to include the header, because the zpool
> will never, ever call zpool->ops->evict
> 
> zpool_shrink()
> this will try to shrink the zpool, using whatever
> zpool-implementation-specific shrinking method.  If zpool_evictable()
> is true for this zpool, then zpool_shrink() *might* callback to
> zpool->ops->evict(), although it doesn't have to if it can shrink
> without evictions.  If zpool_evictable() is false, then zpool_shrink()
> will never callback to zpool->ops->evict().
> 
> There is really no need for zswap to call different functions based on
> whether the pool is evictable or not...is there?

Thanks. I'd prefer if driver drew a clear line between its defrag and
eviction capabilities. But I have no objection to leaving them as its
internal implementation details and keeping current name of
zpool->driver->shrink.

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[PATCH v2] zswap: only save zswap header when necessary

[Yu Zhao]

We waste sizeof(swp_entry_t) for zswap header when using zsmalloc
as zpool driver because zsmalloc doesn't support eviction.

Add zpool_evictable() to detect if zpool is potentially evictable,
and use it in zswap to avoid waste memory for zswap header.

Signed-off-by: Yu Zhao <yuzhao@google.com>
---
 include/linux/zpool.h |  2 ++
 mm/zpool.c            | 26 ++++++++++++++++++++++++--
 mm/zsmalloc.c         |  7 -------
 mm/zswap.c            | 20 ++++++++++----------
 4 files changed, 36 insertions(+), 19 deletions(-)

diff --git a/include/linux/zpool.h b/include/linux/zpool.h
index 004ba807df96..7238865e75b0 100644
--- a/include/linux/zpool.h
+++ b/include/linux/zpool.h
@@ -108,4 +108,6 @@ void zpool_register_driver(struct zpool_driver *driver);
 
 int zpool_unregister_driver(struct zpool_driver *driver);
 
+bool zpool_evictable(struct zpool *pool);
+
 #endif
diff --git a/mm/zpool.c b/mm/zpool.c
index fd3ff719c32c..ec63ef32d73d 100644
--- a/mm/zpool.c
+++ b/mm/zpool.c
@@ -21,6 +21,7 @@ struct zpool {
 	struct zpool_driver *driver;
 	void *pool;
 	const struct zpool_ops *ops;
+	bool evictable;
 
 	struct list_head list;
 };
@@ -142,7 +143,7 @@ EXPORT_SYMBOL(zpool_has_pool);
  *
  * This creates a new zpool of the specified type.  The gfp flags will be
  * used when allocating memory, if the implementation supports it.  If the
- * ops param is NULL, then the created zpool will not be shrinkable.
+ * ops param is NULL, then the created zpool will not be evictable.
  *
  * Implementations must guarantee this to be thread-safe.
  *
@@ -180,6 +181,8 @@ struct zpool *zpool_create_pool(const char *type, const char *name, gfp_t gfp,
 	zpool->driver = driver;
 	zpool->pool = driver->create(name, gfp, ops, zpool);
 	zpool->ops = ops;
+	zpool->evictable = zpool->driver->shrink &&
+			   zpool->ops && zpool->ops->evict;
 
 	if (!zpool->pool) {
 		pr_err("couldn't create %s pool\n", type);
@@ -296,7 +299,8 @@ void zpool_free(struct zpool *zpool, unsigned long handle)
 int zpool_shrink(struct zpool *zpool, unsigned int pages,
 			unsigned int *reclaimed)
 {
-	return zpool->driver->shrink(zpool->pool, pages, reclaimed);
+	return zpool->driver->shrink ?
+	       zpool->driver->shrink(zpool->pool, pages, reclaimed) : -EINVAL;
 }
 
 /**
@@ -355,6 +359,24 @@ u64 zpool_get_total_size(struct zpool *zpool)
 	return zpool->driver->total_size(zpool->pool);
 }
 
+/**
+ * zpool_evictable() - Test if zpool is potentially evictable
+ * @pool	The zpool to test
+ *
+ * Zpool is only potentially evictable when it's created with struct
+ * zpool_ops.evict and its driver implements struct zpool_driver.shrink.
+ *
+ * However, it doesn't necessarily mean driver will use zpool_ops.evict
+ * in its implementation of zpool_driver.shrink. It could do internal
+ * defragmentation instead.
+ *
+ * Returns: true if potentially evictable; false otherwise.
+ */
+bool zpool_evictable(struct zpool *zpool)
+{
+	return zpool->evictable;
+}
+
 MODULE_LICENSE("GPL");
 MODULE_AUTHOR("Dan Streetman <ddstreet@ieee.org>");
 MODULE_DESCRIPTION("Common API for compressed memory storage");
diff --git a/mm/zsmalloc.c b/mm/zsmalloc.c
index 683c0651098c..9cc741bcdb32 100644
--- a/mm/zsmalloc.c
+++ b/mm/zsmalloc.c
@@ -407,12 +407,6 @@ static void zs_zpool_free(void *pool, unsigned long handle)
 	zs_free(pool, handle);
 }
 
-static int zs_zpool_shrink(void *pool, unsigned int pages,
-			unsigned int *reclaimed)
-{
-	return -EINVAL;
-}
-
 static void *zs_zpool_map(void *pool, unsigned long handle,
 			enum zpool_mapmode mm)
 {
@@ -450,7 +444,6 @@ static struct zpool_driver zs_zpool_driver = {
 	.destroy =	zs_zpool_destroy,
 	.malloc =	zs_zpool_malloc,
 	.free =		zs_zpool_free,
-	.shrink =	zs_zpool_shrink,
 	.map =		zs_zpool_map,
 	.unmap =	zs_zpool_unmap,
 	.total_size =	zs_zpool_total_size,
diff --git a/mm/zswap.c b/mm/zswap.c
index d39581a076c3..52a9ab519ab2 100644
--- a/mm/zswap.c
+++ b/mm/zswap.c
@@ -964,11 +964,11 @@ static int zswap_frontswap_store(unsigned type, pgoff_t offset,
 	struct zswap_entry *entry, *dupentry;
 	struct crypto_comp *tfm;
 	int ret;
-	unsigned int dlen = PAGE_SIZE, len;
+	unsigned int hlen, dlen = PAGE_SIZE;
 	unsigned long handle;
 	char *buf;
 	u8 *src, *dst;
-	struct zswap_header *zhdr;
+	struct zswap_header zhdr = { .swpentry = swp_entry(type, offset) };
 
 	if (!zswap_enabled || !tree) {
 		ret = -ENODEV;
@@ -1013,8 +1013,8 @@ static int zswap_frontswap_store(unsigned type, pgoff_t offset,
 	}
 
 	/* store */
-	len = dlen + sizeof(struct zswap_header);
-	ret = zpool_malloc(entry->pool->zpool, len,
+	hlen = zpool_evictable(entry->pool->zpool) ? sizeof(zhdr) : 0;
+	ret = zpool_malloc(entry->pool->zpool, hlen + dlen,
 			   __GFP_NORETRY | __GFP_NOWARN | __GFP_KSWAPD_RECLAIM,
 			   &handle);
 	if (ret == -ENOSPC) {
@@ -1025,10 +1025,9 @@ static int zswap_frontswap_store(unsigned type, pgoff_t offset,
 		zswap_reject_alloc_fail++;
 		goto put_dstmem;
 	}
-	zhdr = zpool_map_handle(entry->pool->zpool, handle, ZPOOL_MM_RW);
-	zhdr->swpentry = swp_entry(type, offset);
-	buf = (u8 *)(zhdr + 1);
-	memcpy(buf, dst, dlen);
+	buf = zpool_map_handle(entry->pool->zpool, handle, ZPOOL_MM_RW);
+	memcpy(buf, &zhdr, hlen);
+	memcpy(buf + hlen, dst, dlen);
 	zpool_unmap_handle(entry->pool->zpool, handle);
 	put_cpu_var(zswap_dstmem);
 
@@ -1091,8 +1090,9 @@ static int zswap_frontswap_load(unsigned type, pgoff_t offset,
 
 	/* decompress */
 	dlen = PAGE_SIZE;
-	src = (u8 *)zpool_map_handle(entry->pool->zpool, entry->handle,
-			ZPOOL_MM_RO) + sizeof(struct zswap_header);
+	src = zpool_map_handle(entry->pool->zpool, entry->handle, ZPOOL_MM_RO);
+	if (zpool_evictable(entry->pool->zpool))
+		src += sizeof(struct zswap_header);
 	dst = kmap_atomic(page);
 	tfm = *get_cpu_ptr(entry->pool->tfm);
 	ret = crypto_comp_decompress(tfm, src, entry->length, dst, &dlen);
-- 
2.16.0.rc1.238.g530d649a79-goog

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[PATCH v3] zswap: only save zswap header when necessary

[Yu Zhao]

We waste sizeof(swp_entry_t) for zswap header when using zsmalloc
as zpool driver because zsmalloc doesn't support eviction.

Add zpool_evictable() to detect if zpool is potentially evictable,
and use it in zswap to avoid waste memory for zswap header.

Signed-off-by: Yu Zhao <yuzhao@google.com>
---
 include/linux/zpool.h |  2 ++
 mm/zpool.c            | 25 +++++++++++++++++++++++--
 mm/zsmalloc.c         |  7 -------
 mm/zswap.c            | 20 ++++++++++----------
 4 files changed, 35 insertions(+), 19 deletions(-)

diff --git a/include/linux/zpool.h b/include/linux/zpool.h
index 004ba807df96..7238865e75b0 100644
--- a/include/linux/zpool.h
+++ b/include/linux/zpool.h
@@ -108,4 +108,6 @@ void zpool_register_driver(struct zpool_driver *driver);
 
 int zpool_unregister_driver(struct zpool_driver *driver);
 
+bool zpool_evictable(struct zpool *pool);
+
 #endif
diff --git a/mm/zpool.c b/mm/zpool.c
index fd3ff719c32c..e1e7aa6d1d06 100644
--- a/mm/zpool.c
+++ b/mm/zpool.c
@@ -21,6 +21,7 @@ struct zpool {
 	struct zpool_driver *driver;
 	void *pool;
 	const struct zpool_ops *ops;
+	bool evictable;
 
 	struct list_head list;
 };
@@ -142,7 +143,7 @@ EXPORT_SYMBOL(zpool_has_pool);
  *
  * This creates a new zpool of the specified type.  The gfp flags will be
  * used when allocating memory, if the implementation supports it.  If the
- * ops param is NULL, then the created zpool will not be shrinkable.
+ * ops param is NULL, then the created zpool will not be evictable.
  *
  * Implementations must guarantee this to be thread-safe.
  *
@@ -180,6 +181,7 @@ struct zpool *zpool_create_pool(const char *type, const char *name, gfp_t gfp,
 	zpool->driver = driver;
 	zpool->pool = driver->create(name, gfp, ops, zpool);
 	zpool->ops = ops;
+	zpool->evictable = driver->shrink && ops && ops->evict;
 
 	if (!zpool->pool) {
 		pr_err("couldn't create %s pool\n", type);
@@ -296,7 +298,8 @@ void zpool_free(struct zpool *zpool, unsigned long handle)
 int zpool_shrink(struct zpool *zpool, unsigned int pages,
 			unsigned int *reclaimed)
 {
-	return zpool->driver->shrink(zpool->pool, pages, reclaimed);
+	return zpool->driver->shrink ?
+	       zpool->driver->shrink(zpool->pool, pages, reclaimed) : -EINVAL;
 }
 
 /**
@@ -355,6 +358,24 @@ u64 zpool_get_total_size(struct zpool *zpool)
 	return zpool->driver->total_size(zpool->pool);
 }
 
+/**
+ * zpool_evictable() - Test if zpool is potentially evictable
+ * @pool	The zpool to test
+ *
+ * Zpool is only potentially evictable when it's created with struct
+ * zpool_ops.evict and its driver implements struct zpool_driver.shrink.
+ *
+ * However, it doesn't necessarily mean driver will use zpool_ops.evict
+ * in its implementation of zpool_driver.shrink. It could do internal
+ * defragmentation instead.
+ *
+ * Returns: true if potentially evictable; false otherwise.
+ */
+bool zpool_evictable(struct zpool *zpool)
+{
+	return zpool->evictable;
+}
+
 MODULE_LICENSE("GPL");
 MODULE_AUTHOR("Dan Streetman <ddstreet@ieee.org>");
 MODULE_DESCRIPTION("Common API for compressed memory storage");
diff --git a/mm/zsmalloc.c b/mm/zsmalloc.c
index 683c0651098c..9cc741bcdb32 100644
--- a/mm/zsmalloc.c
+++ b/mm/zsmalloc.c
@@ -407,12 +407,6 @@ static void zs_zpool_free(void *pool, unsigned long handle)
 	zs_free(pool, handle);
 }
 
-static int zs_zpool_shrink(void *pool, unsigned int pages,
-			unsigned int *reclaimed)
-{
-	return -EINVAL;
-}
-
 static void *zs_zpool_map(void *pool, unsigned long handle,
 			enum zpool_mapmode mm)
 {
@@ -450,7 +444,6 @@ static struct zpool_driver zs_zpool_driver = {
 	.destroy =	zs_zpool_destroy,
 	.malloc =	zs_zpool_malloc,
 	.free =		zs_zpool_free,
-	.shrink =	zs_zpool_shrink,
 	.map =		zs_zpool_map,
 	.unmap =	zs_zpool_unmap,
 	.total_size =	zs_zpool_total_size,
diff --git a/mm/zswap.c b/mm/zswap.c
index d39581a076c3..52a9ab519ab2 100644
--- a/mm/zswap.c
+++ b/mm/zswap.c
@@ -964,11 +964,11 @@ static int zswap_frontswap_store(unsigned type, pgoff_t offset,
 	struct zswap_entry *entry, *dupentry;
 	struct crypto_comp *tfm;
 	int ret;
-	unsigned int dlen = PAGE_SIZE, len;
+	unsigned int hlen, dlen = PAGE_SIZE;
 	unsigned long handle;
 	char *buf;
 	u8 *src, *dst;
-	struct zswap_header *zhdr;
+	struct zswap_header zhdr = { .swpentry = swp_entry(type, offset) };
 
 	if (!zswap_enabled || !tree) {
 		ret = -ENODEV;
@@ -1013,8 +1013,8 @@ static int zswap_frontswap_store(unsigned type, pgoff_t offset,
 	}
 
 	/* store */
-	len = dlen + sizeof(struct zswap_header);
-	ret = zpool_malloc(entry->pool->zpool, len,
+	hlen = zpool_evictable(entry->pool->zpool) ? sizeof(zhdr) : 0;
+	ret = zpool_malloc(entry->pool->zpool, hlen + dlen,
 			   __GFP_NORETRY | __GFP_NOWARN | __GFP_KSWAPD_RECLAIM,
 			   &handle);
 	if (ret == -ENOSPC) {
@@ -1025,10 +1025,9 @@ static int zswap_frontswap_store(unsigned type, pgoff_t offset,
 		zswap_reject_alloc_fail++;
 		goto put_dstmem;
 	}
-	zhdr = zpool_map_handle(entry->pool->zpool, handle, ZPOOL_MM_RW);
-	zhdr->swpentry = swp_entry(type, offset);
-	buf = (u8 *)(zhdr + 1);
-	memcpy(buf, dst, dlen);
+	buf = zpool_map_handle(entry->pool->zpool, handle, ZPOOL_MM_RW);
+	memcpy(buf, &zhdr, hlen);
+	memcpy(buf + hlen, dst, dlen);
 	zpool_unmap_handle(entry->pool->zpool, handle);
 	put_cpu_var(zswap_dstmem);
 
@@ -1091,8 +1090,9 @@ static int zswap_frontswap_load(unsigned type, pgoff_t offset,
 
 	/* decompress */
 	dlen = PAGE_SIZE;
-	src = (u8 *)zpool_map_handle(entry->pool->zpool, entry->handle,
-			ZPOOL_MM_RO) + sizeof(struct zswap_header);
+	src = zpool_map_handle(entry->pool->zpool, entry->handle, ZPOOL_MM_RO);
+	if (zpool_evictable(entry->pool->zpool))
+		src += sizeof(struct zswap_header);
 	dst = kmap_atomic(page);
 	tfm = *get_cpu_ptr(entry->pool->tfm);
 	ret = crypto_comp_decompress(tfm, src, entry->length, dst, &dlen);
-- 
2.16.0.rc1.238.g530d649a79-goog

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Re: [PATCH v2] zswap: only save zswap header when necessary

[Yu Zhao]

On Wed, Jan 10, 2018 at 02:47:41PM -0800, Yu Zhao wrote:
> We waste sizeof(swp_entry_t) for zswap header when using zsmalloc
> as zpool driver because zsmalloc doesn't support eviction.
> 
> Add zpool_evictable() to detect if zpool is potentially evictable,
> and use it in zswap to avoid waste memory for zswap header.
> 
> Signed-off-by: Yu Zhao <yuzhao@google.com>
> ---
>  include/linux/zpool.h |  2 ++
>  mm/zpool.c            | 26 ++++++++++++++++++++++++--
>  mm/zsmalloc.c         |  7 -------
>  mm/zswap.c            | 20 ++++++++++----------
>  4 files changed, 36 insertions(+), 19 deletions(-)
> 
> diff --git a/include/linux/zpool.h b/include/linux/zpool.h
> index 004ba807df96..7238865e75b0 100644
> --- a/include/linux/zpool.h
> +++ b/include/linux/zpool.h
> @@ -108,4 +108,6 @@ void zpool_register_driver(struct zpool_driver *driver);
>  
>  int zpool_unregister_driver(struct zpool_driver *driver);
>  
> +bool zpool_evictable(struct zpool *pool);
> +
>  #endif
> diff --git a/mm/zpool.c b/mm/zpool.c
> index fd3ff719c32c..ec63ef32d73d 100644
> --- a/mm/zpool.c
> +++ b/mm/zpool.c
> @@ -21,6 +21,7 @@ struct zpool {
>  	struct zpool_driver *driver;
>  	void *pool;
>  	const struct zpool_ops *ops;
> +	bool evictable;
>  
>  	struct list_head list;
>  };
> @@ -142,7 +143,7 @@ EXPORT_SYMBOL(zpool_has_pool);
>   *
>   * This creates a new zpool of the specified type.  The gfp flags will be
>   * used when allocating memory, if the implementation supports it.  If the
> - * ops param is NULL, then the created zpool will not be shrinkable.
> + * ops param is NULL, then the created zpool will not be evictable.
>   *
>   * Implementations must guarantee this to be thread-safe.
>   *
> @@ -180,6 +181,8 @@ struct zpool *zpool_create_pool(const char *type, const char *name, gfp_t gfp,
>  	zpool->driver = driver;
>  	zpool->pool = driver->create(name, gfp, ops, zpool);
>  	zpool->ops = ops;
> +	zpool->evictable = zpool->driver->shrink &&
> +			   zpool->ops && zpool->ops->evict;

The zpool->" prefix is a result out copy & paste. Fixed in the next
version. Sorry for the spam.

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Re: [PATCH] zswap: only save zswap header if zpool is shrinkable

[Sergey Senozhatsky]

Hello,

Yu Zhao, Dan, sorry for the delay


On (01/10/18 15:06), Dan Streetman wrote:
[..]
> Well, I think shrink vs evict an implementation detail, isn't it?
> That is, from zswap's perspective, there should be:
> 
> zpool_evictable()
> if true, zswap needs to include the header on each compressed page,
> because the zpool may callback zpool->ops->evict() which calls
> zswap_writeback_entry() which expects the entry to start with a zswap
> header.
> if false, zswap doesn't need to include the header, because the zpool
> will never, ever call zpool->ops->evict
> 
> zpool_shrink()
> this will try to shrink the zpool, using whatever
> zpool-implementation-specific shrinking method.  If zpool_evictable()
> is true for this zpool, then zpool_shrink() *might* callback to
> zpool->ops->evict(), although it doesn't have to if it can shrink
> without evictions.  If zpool_evictable() is false, then zpool_shrink()
> will never callback to zpool->ops->evict().

ACK on this!

	-ss

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Re: [PATCH v3] zswap: only save zswap header when necessary

[Sergey Senozhatsky]

On (01/10/18 14:56), Yu Zhao wrote:
[..]
> We waste sizeof(swp_entry_t) for zswap header when using zsmalloc
> as zpool driver because zsmalloc doesn't support eviction.
>
> Add zpool_evictable() to detect if zpool is potentially evictable,
> and use it in zswap to avoid waste memory for zswap header.
>
> Signed-off-by: Yu Zhao <yuzhao@google.com>

looks good to me.

FWIW,
Reviewed-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>

	-ss

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Re: [PATCH v3] zswap: only save zswap header when necessary

[Dan Streetman]

On Wed, Jan 10, 2018 at 5:56 PM, Yu Zhao <yuzhao@google.com> wrote:
> We waste sizeof(swp_entry_t) for zswap header when using zsmalloc
> as zpool driver because zsmalloc doesn't support eviction.
>
> Add zpool_evictable() to detect if zpool is potentially evictable,
> and use it in zswap to avoid waste memory for zswap header.
>
> Signed-off-by: Yu Zhao <yuzhao@google.com>
> ---
>  include/linux/zpool.h |  2 ++
>  mm/zpool.c            | 25 +++++++++++++++++++++++--
>  mm/zsmalloc.c         |  7 -------
>  mm/zswap.c            | 20 ++++++++++----------
>  4 files changed, 35 insertions(+), 19 deletions(-)
>
> diff --git a/include/linux/zpool.h b/include/linux/zpool.h
> index 004ba807df96..7238865e75b0 100644
> --- a/include/linux/zpool.h
> +++ b/include/linux/zpool.h
> @@ -108,4 +108,6 @@ void zpool_register_driver(struct zpool_driver *driver);
>
>  int zpool_unregister_driver(struct zpool_driver *driver);
>
> +bool zpool_evictable(struct zpool *pool);
> +
>  #endif
> diff --git a/mm/zpool.c b/mm/zpool.c
> index fd3ff719c32c..e1e7aa6d1d06 100644
> --- a/mm/zpool.c
> +++ b/mm/zpool.c
> @@ -21,6 +21,7 @@ struct zpool {
>         struct zpool_driver *driver;
>         void *pool;
>         const struct zpool_ops *ops;
> +       bool evictable;
>
>         struct list_head list;
>  };
> @@ -142,7 +143,7 @@ EXPORT_SYMBOL(zpool_has_pool);
>   *
>   * This creates a new zpool of the specified type.  The gfp flags will be
>   * used when allocating memory, if the implementation supports it.  If the
> - * ops param is NULL, then the created zpool will not be shrinkable.
> + * ops param is NULL, then the created zpool will not be evictable.
>   *
>   * Implementations must guarantee this to be thread-safe.
>   *
> @@ -180,6 +181,7 @@ struct zpool *zpool_create_pool(const char *type, const char *name, gfp_t gfp,
>         zpool->driver = driver;
>         zpool->pool = driver->create(name, gfp, ops, zpool);
>         zpool->ops = ops;
> +       zpool->evictable = driver->shrink && ops && ops->evict;

Since the ops->evict comes from zswap (and is never omitted), if we do
restore zs_zpool_shrink() in the future to call the zsmalloc shrinker
(and not do eviction), we'll have to add a driver->evictable bool to
check here as well.

But for now this is good, the zpools with driver->shrink do eviction,
zsmalloc doesn't do eviction and won't have driver->shrink.

Acked-by: Dan Streetman <ddstreet@ieee.org>

Thanks!

>
>         if (!zpool->pool) {
>                 pr_err("couldn't create %s pool\n", type);
> @@ -296,7 +298,8 @@ void zpool_free(struct zpool *zpool, unsigned long handle)
>  int zpool_shrink(struct zpool *zpool, unsigned int pages,
>                         unsigned int *reclaimed)
>  {
> -       return zpool->driver->shrink(zpool->pool, pages, reclaimed);
> +       return zpool->driver->shrink ?
> +              zpool->driver->shrink(zpool->pool, pages, reclaimed) : -EINVAL;
>  }
>
>  /**
> @@ -355,6 +358,24 @@ u64 zpool_get_total_size(struct zpool *zpool)
>         return zpool->driver->total_size(zpool->pool);
>  }
>
> +/**
> + * zpool_evictable() - Test if zpool is potentially evictable
> + * @pool       The zpool to test
> + *
> + * Zpool is only potentially evictable when it's created with struct
> + * zpool_ops.evict and its driver implements struct zpool_driver.shrink.
> + *
> + * However, it doesn't necessarily mean driver will use zpool_ops.evict
> + * in its implementation of zpool_driver.shrink. It could do internal
> + * defragmentation instead.
> + *
> + * Returns: true if potentially evictable; false otherwise.
> + */
> +bool zpool_evictable(struct zpool *zpool)
> +{
> +       return zpool->evictable;
> +}
> +
>  MODULE_LICENSE("GPL");
>  MODULE_AUTHOR("Dan Streetman <ddstreet@ieee.org>");
>  MODULE_DESCRIPTION("Common API for compressed memory storage");
> diff --git a/mm/zsmalloc.c b/mm/zsmalloc.c
> index 683c0651098c..9cc741bcdb32 100644
> --- a/mm/zsmalloc.c
> +++ b/mm/zsmalloc.c
> @@ -407,12 +407,6 @@ static void zs_zpool_free(void *pool, unsigned long handle)
>         zs_free(pool, handle);
>  }
>
> -static int zs_zpool_shrink(void *pool, unsigned int pages,
> -                       unsigned int *reclaimed)
> -{
> -       return -EINVAL;
> -}
> -
>  static void *zs_zpool_map(void *pool, unsigned long handle,
>                         enum zpool_mapmode mm)
>  {
> @@ -450,7 +444,6 @@ static struct zpool_driver zs_zpool_driver = {
>         .destroy =      zs_zpool_destroy,
>         .malloc =       zs_zpool_malloc,
>         .free =         zs_zpool_free,
> -       .shrink =       zs_zpool_shrink,
>         .map =          zs_zpool_map,
>         .unmap =        zs_zpool_unmap,
>         .total_size =   zs_zpool_total_size,
> diff --git a/mm/zswap.c b/mm/zswap.c
> index d39581a076c3..52a9ab519ab2 100644
> --- a/mm/zswap.c
> +++ b/mm/zswap.c
> @@ -964,11 +964,11 @@ static int zswap_frontswap_store(unsigned type, pgoff_t offset,
>         struct zswap_entry *entry, *dupentry;
>         struct crypto_comp *tfm;
>         int ret;
> -       unsigned int dlen = PAGE_SIZE, len;
> +       unsigned int hlen, dlen = PAGE_SIZE;
>         unsigned long handle;
>         char *buf;
>         u8 *src, *dst;
> -       struct zswap_header *zhdr;
> +       struct zswap_header zhdr = { .swpentry = swp_entry(type, offset) };
>
>         if (!zswap_enabled || !tree) {
>                 ret = -ENODEV;
> @@ -1013,8 +1013,8 @@ static int zswap_frontswap_store(unsigned type, pgoff_t offset,
>         }
>
>         /* store */
> -       len = dlen + sizeof(struct zswap_header);
> -       ret = zpool_malloc(entry->pool->zpool, len,
> +       hlen = zpool_evictable(entry->pool->zpool) ? sizeof(zhdr) : 0;
> +       ret = zpool_malloc(entry->pool->zpool, hlen + dlen,
>                            __GFP_NORETRY | __GFP_NOWARN | __GFP_KSWAPD_RECLAIM,
>                            &handle);
>         if (ret == -ENOSPC) {
> @@ -1025,10 +1025,9 @@ static int zswap_frontswap_store(unsigned type, pgoff_t offset,
>                 zswap_reject_alloc_fail++;
>                 goto put_dstmem;
>         }
> -       zhdr = zpool_map_handle(entry->pool->zpool, handle, ZPOOL_MM_RW);
> -       zhdr->swpentry = swp_entry(type, offset);
> -       buf = (u8 *)(zhdr + 1);
> -       memcpy(buf, dst, dlen);
> +       buf = zpool_map_handle(entry->pool->zpool, handle, ZPOOL_MM_RW);
> +       memcpy(buf, &zhdr, hlen);
> +       memcpy(buf + hlen, dst, dlen);
>         zpool_unmap_handle(entry->pool->zpool, handle);
>         put_cpu_var(zswap_dstmem);
>
> @@ -1091,8 +1090,9 @@ static int zswap_frontswap_load(unsigned type, pgoff_t offset,
>
>         /* decompress */
>         dlen = PAGE_SIZE;
> -       src = (u8 *)zpool_map_handle(entry->pool->zpool, entry->handle,
> -                       ZPOOL_MM_RO) + sizeof(struct zswap_header);
> +       src = zpool_map_handle(entry->pool->zpool, entry->handle, ZPOOL_MM_RO);
> +       if (zpool_evictable(entry->pool->zpool))
> +               src += sizeof(struct zswap_header);
>         dst = kmap_atomic(page);
>         tfm = *get_cpu_ptr(entry->pool->tfm);
>         ret = crypto_comp_decompress(tfm, src, entry->length, dst, &dlen);
> --
> 2.16.0.rc1.238.g530d649a79-goog
>

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