Re: [RFC mm/zswap 1/2] mm/zswap: add the flag can_sleep_mapped

[Vitaly Wool <vitaly.wool@konsulko.com>]

On Thu, Jan 14, 2021 at 7:56 PM Minchan Kim <minchan@kernel.org> wrote:
>
> On Thu, Jan 14, 2021 at 07:40:50PM +0100, Vitaly Wool wrote:
> > On Thu, Jan 14, 2021 at 7:29 PM Minchan Kim <minchan@kernel.org> wrote:
> > >
> > > On Fri, Dec 25, 2020 at 07:02:50PM +0800, Tian Tao wrote:
> > > > add a flag to zpool, named is "can_sleep_mapped", and have it set true
> > > > for zbud/z3fold, set false for zsmalloc. Then zswap could go the current
> > > > path if the flag is true; and if it's false, copy data from src to a
> > > > temporary buffer, then unmap the handle, take the mutex, process the
> > > > buffer instead of src to avoid sleeping function called from atomic
> > > > context.
> > > >
> > > > Signed-off-by: Tian Tao <tiantao6@hisilicon.com>
> > > > ---
> > > >  include/linux/zpool.h |  3 +++
> > > >  mm/zpool.c            | 13 +++++++++++++
> > > >  mm/zswap.c            | 50 +++++++++++++++++++++++++++++++++++++++++++++-----
> > > >  3 files changed, 61 insertions(+), 5 deletions(-)
> > > >
> > > > diff --git a/include/linux/zpool.h b/include/linux/zpool.h
> > > > index 51bf430..e899701 100644
> > > > --- a/include/linux/zpool.h
> > > > +++ b/include/linux/zpool.h
> > > > @@ -73,6 +73,7 @@ u64 zpool_get_total_size(struct zpool *pool);
> > > >   * @malloc:  allocate mem from a pool.
> > > >   * @free:    free mem from a pool.
> > > >   * @shrink:  shrink the pool.
> > > > + * @sleep_mapped: whether zpool driver can sleep during map.
> > >
> > > I don't think it's a good idea. It just breaks zpool abstraction
> > > in that it exposes internal implementation to user to avoid issue
> > > zswap recently introduced. It also conflicts zpool_map_handle's
> > > semantic.
> > >
> > > Rather than introducing another break in zpool due to the new
> > > zswap feature recenlty introduced, zswap could introduce
> > > CONFIG_ZSWAP_HW_COMPRESSOR. Once it's configured, zsmalloc could
> > > be disabled. And with disabling CONFIG_ZSWAP_HW_COMPRESSOR, zswap
> > > doesn't need to make any bounce buffer copy so that no existing
> > > zsmalloc user will see performance regression.
> >
> > I believe it won't help that much -- the new compressor API presumes
> > that the caller may sleep during compression and that will be an
> > accident waiting to happen as long as we use it and keep the handle
> > mapped in zsmalloc case.
> >
> > Or maybe I interpreted you wrong and you are suggesting re-introducing
> > calls to the old API under this #ifdef, is that the case?
>
> Yub. zswap could abstract that part under #ifdef to keep old behavior.

We can reconsider this option when zsmalloc implements reclaim
callback. So far it's obviously too much a mess for a reason so weak.

> >
> > Best regards,
> >    Vitaly
> >
> > >
> > > >   * @map:     map a handle.
> > > >   * @unmap:   unmap a handle.
> > > >   * @total_size:      get total size of a pool.
> > > > @@ -100,6 +101,7 @@ struct zpool_driver {
> > > >       int (*shrink)(void *pool, unsigned int pages,
> > > >                               unsigned int *reclaimed);
> > > >
> > > > +     bool sleep_mapped;
> > > >       void *(*map)(void *pool, unsigned long handle,
> > > >                               enum zpool_mapmode mm);
> > > >       void (*unmap)(void *pool, unsigned long handle);
> > > > @@ -112,5 +114,6 @@ void zpool_register_driver(struct zpool_driver *driver);
> > > >  int zpool_unregister_driver(struct zpool_driver *driver);
> > > >
> > > >  bool zpool_evictable(struct zpool *pool);
> > > > +bool zpool_can_sleep_mapped(struct zpool *pool);
> > > >
> > > >  #endif
> > > > diff --git a/mm/zpool.c b/mm/zpool.c
> > > > index 3744a2d..5ed7120 100644
> > > > --- a/mm/zpool.c
> > > > +++ b/mm/zpool.c
> > > > @@ -23,6 +23,7 @@ struct zpool {
> > > >       void *pool;
> > > >       const struct zpool_ops *ops;
> > > >       bool evictable;
> > > > +     bool can_sleep_mapped;
> > > >
> > > >       struct list_head list;
> > > >  };
> > > > @@ -183,6 +184,7 @@ struct zpool *zpool_create_pool(const char *type, const char *name, gfp_t gfp,
> > > >       zpool->pool = driver->create(name, gfp, ops, zpool);
> > > >       zpool->ops = ops;
> > > >       zpool->evictable = driver->shrink && ops && ops->evict;
> > > > +     zpool->can_sleep_mapped = driver->sleep_mapped;
> > > >
> > > >       if (!zpool->pool) {
> > > >               pr_err("couldn't create %s pool\n", type);
> > > > @@ -393,6 +395,17 @@ bool zpool_evictable(struct zpool *zpool)
> > > >       return zpool->evictable;
> > > >  }
> > > >
> > > > +/**
> > > > + * zpool_can_sleep_mapped - Test if zpool can sleep when do mapped.
> > > > + * @zpool:   The zpool to test
> > > > + *
> > > > + * Returns: true if zpool can sleep; false otherwise.
> > > > + */
> > > > +bool zpool_can_sleep_mapped(struct zpool *zpool)
> > > > +{
> > > > +     return zpool->can_sleep_mapped;
> > > > +}
> > > > +
> > > >  MODULE_LICENSE("GPL");
> > > >  MODULE_AUTHOR("Dan Streetman <ddstreet@ieee.org>");
> > > >  MODULE_DESCRIPTION("Common API for compressed memory storage");
> > > > diff --git a/mm/zswap.c b/mm/zswap.c
> > > > index 182f6ad..67d4555 100644
> > > > --- a/mm/zswap.c
> > > > +++ b/mm/zswap.c
> > > > @@ -935,13 +935,20 @@ static int zswap_writeback_entry(struct zpool *pool, unsigned long handle)
> > > >       struct scatterlist input, output;
> > > >       struct crypto_acomp_ctx *acomp_ctx;
> > > >
> > > > -     u8 *src;
> > > > +     u8 *src, *tmp;
> > > >       unsigned int dlen;
> > > >       int ret;
> > > >       struct writeback_control wbc = {
> > > >               .sync_mode = WB_SYNC_NONE,
> > > >       };
> > > >
> > > > +     if (!zpool_can_sleep_mapped(pool)) {
> > > > +
> > > > +             tmp = kmalloc(entry->length, GFP_ATOMIC);
> > > > +             if (!tmp)
> > > > +                     return -ENOMEM;
> > > > +     }
> > > > +
> > > >       /* extract swpentry from data */
> > > >       zhdr = zpool_map_handle(pool, handle, ZPOOL_MM_RO);
> > > >       swpentry = zhdr->swpentry; /* here */
> > > > @@ -979,6 +986,14 @@ static int zswap_writeback_entry(struct zpool *pool, unsigned long handle)
> > > >               dlen = PAGE_SIZE;
> > > >               src = (u8 *)zhdr + sizeof(struct zswap_header);
> > > >
> > > > +             if (!zpool_can_sleep_mapped(pool)) {
> > > > +
> > > > +                     memcpy(tmp, src, entry->length);
> > > > +                     src = tmp;
> > > > +
> > > > +                     zpool_unmap_handle(pool, handle);
> > > > +             }
> > > > +
> > > >               mutex_lock(acomp_ctx->mutex);
> > > >               sg_init_one(&input, src, entry->length);
> > > >               sg_init_table(&output, 1);
> > > > @@ -1033,7 +1048,11 @@ static int zswap_writeback_entry(struct zpool *pool, unsigned long handle)
> > > >       spin_unlock(&tree->lock);
> > > >
> > > >  end:
> > > > -     zpool_unmap_handle(pool, handle);
> > > > +     if (zpool_can_sleep_mapped(pool))
> > > > +             zpool_unmap_handle(pool, handle);
> > > > +     else
> > > > +             kfree(tmp);
> > > > +
> > > >       return ret;
> > > >  }
> > > >
> > > > @@ -1235,7 +1254,7 @@ static int zswap_frontswap_load(unsigned type, pgoff_t offset,
> > > >       struct zswap_entry *entry;
> > > >       struct scatterlist input, output;
> > > >       struct crypto_acomp_ctx *acomp_ctx;
> > > > -     u8 *src, *dst;
> > > > +     u8 *src, *dst, *tmp;
> > > >       unsigned int dlen;
> > > >       int ret;
> > > >
> > > > @@ -1256,12 +1275,29 @@ static int zswap_frontswap_load(unsigned type, pgoff_t offset,
> > > >               goto freeentry;
> > > >       }
> > > >
> > > > +     if (!zpool_can_sleep_mapped(entry->pool->zpool)) {
> > > > +
> > > > +             tmp = kmalloc(entry->length, GFP_ATOMIC);
> > > > +             if (!tmp) {
> > > > +                     ret = -ENOMEM;
> > > > +                     goto freeentry;
> > > > +             }
> > > > +     }
> > > > +
> > > >       /* decompress */
> > > >       dlen = PAGE_SIZE;
> > > >       src = zpool_map_handle(entry->pool->zpool, entry->handle, ZPOOL_MM_RO);
> > > >       if (zpool_evictable(entry->pool->zpool))
> > > >               src += sizeof(struct zswap_header);
> > > >
> > > > +     if (!zpool_can_sleep_mapped(entry->pool->zpool)) {
> > > > +
> > > > +             memcpy(tmp, src, entry->length);
> > > > +             src = tmp;
> > > > +
> > > > +             zpool_unmap_handle(entry->pool->zpool, entry->handle);
> > > > +     }
> > > > +
> > > >       acomp_ctx = raw_cpu_ptr(entry->pool->acomp_ctx);
> > > >       mutex_lock(acomp_ctx->mutex);
> > > >       sg_init_one(&input, src, entry->length);
> > > > @@ -1271,7 +1307,11 @@ static int zswap_frontswap_load(unsigned type, pgoff_t offset,
> > > >       ret = crypto_wait_req(crypto_acomp_decompress(acomp_ctx->req), &acomp_ctx->wait);
> > > >       mutex_unlock(acomp_ctx->mutex);
> > > >
> > > > -     zpool_unmap_handle(entry->pool->zpool, entry->handle);
> > > > +     if (zpool_can_sleep_mapped(entry->pool->zpool))
> > > > +             zpool_unmap_handle(entry->pool->zpool, entry->handle);
> > > > +     else
> > > > +             kfree(tmp);
> > > > +
> > > >       BUG_ON(ret);
> > > >
> > > >  freeentry:
> > > > @@ -1279,7 +1319,7 @@ static int zswap_frontswap_load(unsigned type, pgoff_t offset,
> > > >       zswap_entry_put(tree, entry);
> > > >       spin_unlock(&tree->lock);
> > > >
> > > > -     return 0;
> > > > +     return ret;
> > > >  }
> > > >
> > > >  /* frees an entry in zswap */
> > > > --
> > > > 2.7.4
> > > >
> > > >