[PATCH] mm/fadvise: discard partial pages iff endbyte is also eof

[PATCH] mm/fadvise: discard partial pages iff endbyte is also eof

[十刀]

From: "shidao.ytt" <shidao.ytt@alibaba-inc.com>

in commit 441c228f817f7 ("mm: fadvise: document the
fadvise(FADV_DONTNEED) behaviour for partial pages") Mel Gorman
explained why partial pages should be preserved instead of discarded
when using fadvise(FADV_DONTNEED), however the actual codes to calcuate
end_index was unexpectedly wrong, the code behavior didn't match to the
statement in comments; Luckily in another commit 18aba41cbf
("mm/fadvise.c: do not discard partial pages with POSIX_FADV_DONTNEED")
Oleg Drokin fixed this behavior

Here I come up with a new idea that actually we can still discard the
last parital page iff the page-unaligned endbyte is also the end of
file, since no one else will use the rest of the page and it should be
safe enough to discard.

Signed-off-by: shidao.ytt <shidao.ytt@alibaba-inc.com>
Signed-off-by: Caspar Zhang <jinli.zjl@alibaba-inc.com>
---
 mm/fadvise.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/mm/fadvise.c b/mm/fadvise.c
index ec70d6e..f74b21e 100644
--- a/mm/fadvise.c
+++ b/mm/fadvise.c
@@ -127,7 +127,8 @@
 		 */
 		start_index = (offset+(PAGE_SIZE-1)) >> PAGE_SHIFT;
 		end_index = (endbyte >> PAGE_SHIFT);
-		if ((endbyte & ~PAGE_MASK) != ~PAGE_MASK) {
+		if ((endbyte & ~PAGE_MASK) != ~PAGE_MASK &&
+				endbyte != inode->i_size - 1) {
 			/* First page is tricky as 0 - 1 = -1, but pgoff_t
 			 * is unsigned, so the end_index >= start_index
 			 * check below would be true and we'll discard the whole
-- 
1.8.3.1

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Re: [PATCH] mm/fadvise: discard partial pages iff endbyte is also eof

[夷则(Caspar)]

> 在 2017年12月23日，12:16，十刀 <shidao.ytt@alibaba-inc.com> 写道：
> 
> From: "shidao.ytt" <shidao.ytt@alibaba-inc.com>
> 
> in commit 441c228f817f7 ("mm: fadvise: document the
> fadvise(FADV_DONTNEED) behaviour for partial pages") Mel Gorman
> explained why partial pages should be preserved instead of discarded
> when using fadvise(FADV_DONTNEED), however the actual codes to calcuate
> end_index was unexpectedly wrong, the code behavior didn't match to the
> statement in comments; Luckily in another commit 18aba41cbf
> ("mm/fadvise.c: do not discard partial pages with POSIX_FADV_DONTNEED")
> Oleg Drokin fixed this behavior
> 
> Here I come up with a new idea that actually we can still discard the
> last parital page iff the page-unaligned endbyte is also the end of
> file, since no one else will use the rest of the page and it should be
> safe enough to discard.

+akpm...

Hi Mel, Andrew:

Would you please take a look at this patch, to see if this proposal
is reasonable enough, thanks in advance!

Thanks,
Caspar

> 
> Signed-off-by: shidao.ytt <shidao.ytt@alibaba-inc.com>
> Signed-off-by: Caspar Zhang <jinli.zjl@alibaba-inc.com>
> ---
> mm/fadvise.c | 3 ++-
> 1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/mm/fadvise.c b/mm/fadvise.c
> index ec70d6e..f74b21e 100644
> --- a/mm/fadvise.c
> +++ b/mm/fadvise.c
> @@ -127,7 +127,8 @@
> 		 */
> 		start_index = (offset+(PAGE_SIZE-1)) >> PAGE_SHIFT;
> 		end_index = (endbyte >> PAGE_SHIFT);
> -		if ((endbyte & ~PAGE_MASK) != ~PAGE_MASK) {
> +		if ((endbyte & ~PAGE_MASK) != ~PAGE_MASK &&
> +				endbyte != inode->i_size - 1) {
> 			/* First page is tricky as 0 - 1 = -1, but pgoff_t
> 			 * is unsigned, so the end_index >= start_index
> 			 * check below would be true and we'll discard the whole
> -- 
> 1.8.3.1

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Re: [PATCH] mm/fadvise: discard partial pages iff endbyte is also eof

[Mel Gorman]

On Wed, Jan 03, 2018 at 02:53:43PM +0800, ??????(Caspar) wrote:
> 
> 
> > ?? 2017??12??23????12:16?????? <shidao.ytt@alibaba-inc.com> ??????
> > 
> > From: "shidao.ytt" <shidao.ytt@alibaba-inc.com>
> > 
> > in commit 441c228f817f7 ("mm: fadvise: document the
> > fadvise(FADV_DONTNEED) behaviour for partial pages") Mel Gorman
> > explained why partial pages should be preserved instead of discarded
> > when using fadvise(FADV_DONTNEED), however the actual codes to calcuate
> > end_index was unexpectedly wrong, the code behavior didn't match to the
> > statement in comments; Luckily in another commit 18aba41cbf
> > ("mm/fadvise.c: do not discard partial pages with POSIX_FADV_DONTNEED")
> > Oleg Drokin fixed this behavior
> > 
> > Here I come up with a new idea that actually we can still discard the
> > last parital page iff the page-unaligned endbyte is also the end of
> > file, since no one else will use the rest of the page and it should be
> > safe enough to discard.
> 
> +akpm...
> 
> Hi Mel, Andrew:
> 
> Would you please take a look at this patch, to see if this proposal
> is reasonable enough, thanks in advance!
> 

I'm backlogged after being out for the Christmas. Superficially the patch
looks ok but I wondered how often it happened in practice as we already
would discard files smaller than a page on DONTNEED. It also requires
that the system call get the exact size of the file correct and would not
discard if the off + len was past the end of the file for whatever reason
(e.g. a stat to read the size, a truncate in parallel and fadvise using
stale data from stat) and that's why the patch looked like it might have
no impact in practice. Is the patch known to help a real workload or is
it motivated by a code inspection?

-- 
Mel Gorman
SUSE Labs

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Re: [PATCH] mm/fadvise: discard partial pages iff endbyte is also eof

[Andrew Morton]

On Wed, 3 Jan 2018 10:48:00 +0000 Mel Gorman <mgorman@techsingularity.net> wrote:

> On Wed, Jan 03, 2018 at 02:53:43PM +0800, ??????(Caspar) wrote:
> > 
> > 
> > > ?? 2017??12??23????12:16?????? <shidao.ytt@alibaba-inc.com> ??????
> > > 
> > > From: "shidao.ytt" <shidao.ytt@alibaba-inc.com>
> > > 
> > > in commit 441c228f817f7 ("mm: fadvise: document the
> > > fadvise(FADV_DONTNEED) behaviour for partial pages") Mel Gorman
> > > explained why partial pages should be preserved instead of discarded
> > > when using fadvise(FADV_DONTNEED), however the actual codes to calcuate
> > > end_index was unexpectedly wrong, the code behavior didn't match to the
> > > statement in comments; Luckily in another commit 18aba41cbf
> > > ("mm/fadvise.c: do not discard partial pages with POSIX_FADV_DONTNEED")
> > > Oleg Drokin fixed this behavior
> > > 
> > > Here I come up with a new idea that actually we can still discard the
> > > last parital page iff the page-unaligned endbyte is also the end of
> > > file, since no one else will use the rest of the page and it should be
> > > safe enough to discard.
> > 
> > +akpm...
> > 
> > Hi Mel, Andrew:
> > 
> > Would you please take a look at this patch, to see if this proposal
> > is reasonable enough, thanks in advance!
> > 
> 
> I'm backlogged after being out for the Christmas. Superficially the patch
> looks ok but I wondered how often it happened in practice as we already
> would discard files smaller than a page on DONTNEED. It also requires
> that the system call get the exact size of the file correct and would not
> discard if the off + len was past the end of the file for whatever reason
> (e.g. a stat to read the size, a truncate in parallel and fadvise using
> stale data from stat) and that's why the patch looked like it might have
> no impact in practice. Is the patch known to help a real workload or is
> it motivated by a code inspection?

The current whole-pages-only logic was introduced (accidentally, I
think) by yours truly when fixing a bug in the initial fadvise()
commit in 2003. 

https://kernel.opensuse.org/cgit/kernel/commit/?h=v2.6.0-test4&id=7161ee20fea6e25a32feb91503ca2b7c7333c886

Namely:

: invalidate_mapping_pages() takes start/end, but fadvise is currently passing
: it start/len.
: 
: 
: 
:  mm/fadvise.c |    8 ++++++--
:  1 files changed, 6 insertions(+), 2 deletions(-)
: 
: diff -puN mm/fadvise.c~fadvise-fix mm/fadvise.c
: --- 25/mm/fadvise.c~fadvise-fix	2003-08-14 18:16:12.000000000 -0700
: +++ 25-akpm/mm/fadvise.c	2003-08-14 18:16:12.000000000 -0700
: @@ -26,6 +26,8 @@ long sys_fadvise64(int fd, loff_t offset
:  	struct inode *inode;
:  	struct address_space *mapping;
:  	struct backing_dev_info *bdi;
: +	pgoff_t start_index;
: +	pgoff_t end_index;
:  	int ret = 0;
:  
:  	if (!file)
: @@ -65,8 +67,10 @@ long sys_fadvise64(int fd, loff_t offset
:  	case POSIX_FADV_DONTNEED:
:  		if (!bdi_write_congested(mapping->backing_dev_info))
:  			filemap_flush(mapping);
: -		invalidate_mapping_pages(mapping, offset >> PAGE_CACHE_SHIFT,
: -				(len >> PAGE_CACHE_SHIFT) + 1);
: +		start_index = offset >> PAGE_CACHE_SHIFT;
: +		end_index = (offset + len + PAGE_CACHE_SIZE - 1) >>
: +						PAGE_CACHE_SHIFT;
: +		invalidate_mapping_pages(mapping, start_index, end_index);
:  		break;
:  	default:
:  		ret = -EINVAL;
: 

So I'm not sure that the whole "don't discard partial pages" thing is
well-founded and I see no reason why we cannot alter it.

So, thinking caps on: why not just discard them?  After all, that's
what userspace asked us to do.

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Re: [PATCH] mm/fadvise: discard partial pages iff endbyte is also eof

[夷则(Caspar)]

On 2018/1/3 18:48, Mel Gorman wrote:
> On Wed, Jan 03, 2018 at 02:53:43PM +0800, ??????(Caspar) wrote:
>>
>>
>>> ?? 2017??12??23????12:16?????? <shidao.ytt@alibaba-inc.com> ??????
>>>
>>> From: "shidao.ytt" <shidao.ytt@alibaba-inc.com>
>>>
>>> in commit 441c228f817f7 ("mm: fadvise: document the
>>> fadvise(FADV_DONTNEED) behaviour for partial pages") Mel Gorman
>>> explained why partial pages should be preserved instead of discarded
>>> when using fadvise(FADV_DONTNEED), however the actual codes to calcuate
>>> end_index was unexpectedly wrong, the code behavior didn't match to the
>>> statement in comments; Luckily in another commit 18aba41cbf
>>> ("mm/fadvise.c: do not discard partial pages with POSIX_FADV_DONTNEED")
>>> Oleg Drokin fixed this behavior
>>>
>>> Here I come up with a new idea that actually we can still discard the
>>> last parital page iff the page-unaligned endbyte is also the end of
>>> file, since no one else will use the rest of the page and it should be
>>> safe enough to discard.
>>
>> +akpm...
>>
>> Hi Mel, Andrew:
>>
>> Would you please take a look at this patch, to see if this proposal
>> is reasonable enough, thanks in advance!
>>
> 
> I'm backlogged after being out for the Christmas. Superficially the patch
> looks ok but I wondered how often it happened in practice as we already
> would discard files smaller than a page on DONTNEED. It also requires

Actually, we would *not*. Let's look into the codes.

Clue 1: start_index is a round-up page-aligned addr and end_index is a 
round-down page-aligned addr, while offset & endbyte might be unaligned 
obviously (mm/fadvise.c):

       start_index = (offset+(PAGE_SIZE-1)) >> PAGE_SHIFT;
       end_index = (endbyte >> PAGE_SHIFT);
       if ((endbyte & ~PAGE_MASK) != ~PAGE_MASK) {
          /* First page is tricky as 0 - 1 = -1, but pgoff_t
           * is unsigned, so the end_index >= start_index
           * check below would be true and we'll discard the whole
           * file cache which is not what was asked.
           */
          if (end_index == 0)
             break;

          end_index--;
       }

       if (end_index >= start_index) {
          <snip>
          count = invalidate_mapping_pages(mapping,
                   start_index, end_index);
          <snip>


Clue 2: looking into invalidate_mapping_pages() definition in 
mm/truncate.c, we see the end_index is included:

     * @end: the offset 'to' which to invalidate (inclusive)

Now we know:

+ if `offset' is an unaligned addr, the start partial page will not be 
discarded,
+ if `endbyte' is not aligned: behaviors before and after commit 
18aba41cbf ("mm/fadvise.c: do not discard partial pages with 
POSIX_FADV_DONTNEED") are different.
  + before: end_index is the start addr of the last partial page, thus 
the whole page will be invalidated according to 
invalidate_mapping_page() comments and implementation;
  + after: in commit 18aba41cbf, `endbyte' gets checked again, if it is 
not aligned, draw back by one page so that the partial page would not be 
included; and a special case is that if end_index == 0, means the length 
mapped less than a single page, the code just breaks and never runs into 
invalidate_mapping_pages().

We have done some experiments based on an opensource tool vmtouch[1], to 
simplify the reproducer, I also make a simple .c program [2]. Here is 
the output:

* Test 1, upstream:

[root@caspar ~]# uname -r
4.15.0-rc6+

[root@caspar ~]# dd if=/dev/zero of=testfile_1k bs=1k count=1
1+0 records in
1+0 records out
1024 bytes (1.0 kB) copied, 0.000852646 s, 1.2 MB/s

[root@caspar ~]# ./test_fadvise testfile_1k
file size: 1024 Bytes
length of pages: 1
start addr of mmap: 0x7f7aa0f1b000
do posix_fadvise(DONTNEED)
still resident in memory?
vec[0]: 1

[root@caspar ~]# dd if=/dev/zero of=testfile_10k bs=1k count=10
10+0 records in
10+0 records out
10240 bytes (10 kB) copied, 0.000931652 s, 11.0 MB/s

[root@caspar ~]# ./test_fadvise testfile_10k
file size: 10240 Bytes
length of pages: 3
start addr of mmap: 0x7ff57de8f000
do posix_fadvise(DONTNEED)
still resident in memory?
vec[0]: 0
vec[1]: 0
vec[2]: 1

* Test 2, reverted 18aba41cbf

[root@caspar ~]# uname -r
4.15.0-rc6.revert+

[root@caspar ~]# dd if=/dev/zero of=testfile_10k bs=1k count=1
1+0 records in
1+0 records out
1024 bytes (1.0 kB) copied, 0.000858957 s, 1.2 MB/s

[root@caspar ~]# ./test_fadvise testfile_1k
file size: 1024 Bytes
length of pages: 1
start addr of mmap: 0x7f07fe08b000
do posix_fadvise(DONTNEED)
still resident in memory?
vec[0]: 0

[root@caspar ~]# dd if=/dev/zero of=testfile_10k bs=1k count=10
10+0 records in
10+0 records out
10240 bytes (10 kB) copied, 0.00083475 s, 12.3 MB/s
[root@caspar ~]# ./test_fadvise testfile_10k
file size: 10240 Bytes
length of pages: 3
start addr of mmap: 0x7f6a49541000
do posix_fadvise(DONTNEED)
still resident in memory?
vec[0]: 0
vec[1]: 0
vec[2]: 0

* Test 3, patched with our original proposal

[root@caspar ~]# uname -r
4.15.0-rc6.patched+

[root@caspar ~]# dd if=/dev/zero of=testfile_1k bs=1k count=1
1+0 records in
1+0 records out
1024 bytes (1.0 kB) copied, 0.000852275 s, 1.2 MB/s

[root@caspar ~]# ./test_fadvise testfile_1k
file size: 1024 Bytes
length of pages: 1
start addr of mmap: 0x7f0ef6407000
do posix_fadvise(DONTNEED)
still resident in memory?
vec[0]: 0

[root@caspar ~]# dd if=/dev/zero of=testfile_10k bs=1k count=10
10+0 records in
10+0 records out
10240 bytes (10 kB) copied, 0.000939927 s, 10.9 MB/s

[root@caspar ~]# ./test_fadvise testfile_10k
file size: 10240 Bytes
length of pages: 3
start addr of mmap: 0x7f9fb70f1000
do posix_fadvise(DONTNEED)
still resident in memory?
vec[0]: 0
vec[1]: 0
vec[2]: 0

Our analysis matches what we observed from the output, in the latest 
upstream codes, none partial pages would be discarded even it's the 
everything of a less-than-4k file.

> that the system call get the exact size of the file correct and would not
> discard if the off + len was past the end of the file for whatever reason
> (e.g. a stat to read the size, a truncate in parallel and fadvise using
> stale data from stat) and that's why the patch looked like it might have
> no impact in practice. Is the patch known to help a real workload or is
> it motivated by a code inspection?

This patch is trying to help to solve a real issue. Sometimes we need to 
evict the whole file from page cache because we are sure it will not be 
used in the near future. We try to use posix_fadvise() to finish our 
work but we often see a "small tail" at the end of some files could not 
be evicted, after digging a little bit, we find those file sizes are not 
page-aligned and the "tail" turns out to be partial pages.

We fail to find a standard from posix_fadvise() manual page to subscribe 
the function behaviors if the `offset' and `len' params are not 
page-aligned, then we go to kernel tree and see this:

         /*
          * First and last FULL page! Partial pages are deliberately
          * preserved on the expectation that it is better to preserve
          * needed memory than to discard unneeded memory.
          */

So we know the left of partial page is most likely to be a by-design 
behavior, but is it really necessary when it is at the end of the file? 
We know that the rest of the partial page will be unlikely to be used 
and it should be safe enough to discard the whole page, thus, here is 
the patch.

Comments?

Thanks,
Caspar

> 

[1] https://hoytech.com/vmtouch/

[2] reproducer (test_fadvise.c):

#include <sys/mman.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <stdlib.h>
#include <string.h>
#include <stdio.h>
#include <unistd.h>


int main(int argc, char **argv)
{
	int i, fd, ret, len;
	struct stat buf;
	void *addr;
	unsigned char *vec;
	ssize_t pagesize = getpagesize();

	fd = open(argv[1], O_RDWR);
	if (fd < 0)
		return -1;

	ret = fstat(fd, &buf);
	if (ret < 0)
		return -1;
	printf("file size: %u Bytes\n", buf.st_size);

	len = (buf.st_size + pagesize - 1) / pagesize;
	printf("length of pages: %d\n", len);

	addr = mmap(NULL, buf.st_size, PROT_READ, MAP_SHARED, fd, 0);
	if (addr == MAP_FAILED)
		return -1;
	printf("start addr of mmap: %p\n", addr);

	ret = posix_fadvise(fd, 0, buf.st_size, POSIX_FADV_DONTNEED);
	if (ret < 0)
		return -1;

	printf("still resident in memory?\n");
	vec = malloc(len);
	ret = mincore(addr, buf.st_size, (void *)vec);
	if (ret < 0)
		return -1;
	for (i = 0; i < len; i++)
		printf("vec[%d]: %x\n", i, vec[i] & 0x1);
	free(vec);
	
	close(fd);
	return 0;
}

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Re: [PATCH] mm/fadvise: discard partial pages iff endbyte is also eof

[夷则(Caspar)]

On 2018/1/4 14:13, a?.a??(Caspar) wrote:
> 
> This patch is trying to help to solve a real issue. Sometimes we need to 
> evict the whole file from page cache because we are sure it will not be 
> used in the near future. We try to use posix_fadvise() to finish our 
> work but we often see a "small tail" at the end of some files could not 
> be evicted, after digging a little bit, we find those file sizes are not 
> page-aligned and the "tail" turns out to be partial pages.
> 
> We fail to find a standard from posix_fadvise() manual page to subscribe 
> the function behaviors if the `offset' and `len' params are not 

Oops, I find a 'standard' documented in latest man-pages.git[1], blame 
my centos7, it runs with an old man-pages.rpm :-(

Thanks,
Caspar

[1] 
https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/commit/?h=ceb1c326b9f3e863dfd9bf33bc7118bb1fa29bfc

> page-aligned, then we go to kernel tree and see this:
> 
>  A A A A A A A  /*
>  A A A A A A A A  * First and last FULL page! Partial pages are deliberately
>  A A A A A A A A  * preserved on the expectation that it is better to preserve
>  A A A A A A A A  * needed memory than to discard unneeded memory.
>  A A A A A A A A  */

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Re: [PATCH] mm/fadvise: discard partial pages iff endbyte is also eof

[夷则(Caspar)]

On 2018/1/4 08:17, Andrew Morton wrote:
> On Wed, 3 Jan 2018 10:48:00 +0000 Mel Gorman <mgorman@techsingularity.net> wrote:
> 
>> On Wed, Jan 03, 2018 at 02:53:43PM +0800, ??????(Caspar) wrote:
>>>
>>>
>>>> ?? 2017??12??23????12:16?????? <shidao.ytt@alibaba-inc.com> ??????
>>>>
>>>> From: "shidao.ytt" <shidao.ytt@alibaba-inc.com>
>>>>
>>>> in commit 441c228f817f7 ("mm: fadvise: document the
>>>> fadvise(FADV_DONTNEED) behaviour for partial pages") Mel Gorman
>>>> explained why partial pages should be preserved instead of discarded
>>>> when using fadvise(FADV_DONTNEED), however the actual codes to calcuate
>>>> end_index was unexpectedly wrong, the code behavior didn't match to the
>>>> statement in comments; Luckily in another commit 18aba41cbf
>>>> ("mm/fadvise.c: do not discard partial pages with POSIX_FADV_DONTNEED")
>>>> Oleg Drokin fixed this behavior
>>>>
>>>> Here I come up with a new idea that actually we can still discard the
>>>> last parital page iff the page-unaligned endbyte is also the end of
>>>> file, since no one else will use the rest of the page and it should be
>>>> safe enough to discard.
>>>
>>> +akpm...
>>>
>>> Hi Mel, Andrew:
>>>
>>> Would you please take a look at this patch, to see if this proposal
>>> is reasonable enough, thanks in advance!
>>>
>>
>> I'm backlogged after being out for the Christmas. Superficially the patch
>> looks ok but I wondered how often it happened in practice as we already
>> would discard files smaller than a page on DONTNEED. It also requires
>> that the system call get the exact size of the file correct and would not
>> discard if the off + len was past the end of the file for whatever reason
>> (e.g. a stat to read the size, a truncate in parallel and fadvise using
>> stale data from stat) and that's why the patch looked like it might have
>> no impact in practice. Is the patch known to help a real workload or is
>> it motivated by a code inspection?
> 
> The current whole-pages-only logic was introduced (accidentally, I
> think) by yours truly when fixing a bug in the initial fadvise()
> commit in 2003.
> 
> https://kernel.opensuse.org/cgit/kernel/commit/?h=v2.6.0-test4&id=7161ee20fea6e25a32feb91503ca2b7c7333c886
> 
> Namely:
> 
> : invalidate_mapping_pages() takes start/end, but fadvise is currently passing
> : it start/len.
> :
> :
> :
> :  mm/fadvise.c |    8 ++++++--
> :  1 files changed, 6 insertions(+), 2 deletions(-)
> :
> : diff -puN mm/fadvise.c~fadvise-fix mm/fadvise.c
> : --- 25/mm/fadvise.c~fadvise-fix	2003-08-14 18:16:12.000000000 -0700
> : +++ 25-akpm/mm/fadvise.c	2003-08-14 18:16:12.000000000 -0700
> : @@ -26,6 +26,8 @@ long sys_fadvise64(int fd, loff_t offset
> :  	struct inode *inode;
> :  	struct address_space *mapping;
> :  	struct backing_dev_info *bdi;
> : +	pgoff_t start_index;
> : +	pgoff_t end_index;
> :  	int ret = 0;
> :
> :  	if (!file)
> : @@ -65,8 +67,10 @@ long sys_fadvise64(int fd, loff_t offset
> :  	case POSIX_FADV_DONTNEED:
> :  		if (!bdi_write_congested(mapping->backing_dev_info))
> :  			filemap_flush(mapping);
> : -		invalidate_mapping_pages(mapping, offset >> PAGE_CACHE_SHIFT,
> : -				(len >> PAGE_CACHE_SHIFT) + 1);
> : +		start_index = offset >> PAGE_CACHE_SHIFT;
> : +		end_index = (offset + len + PAGE_CACHE_SIZE - 1) >>
> : +						PAGE_CACHE_SHIFT;
> : +		invalidate_mapping_pages(mapping, start_index, end_index);
> :  		break;
> :  	default:
> :  		ret = -EINVAL;
> :
> 
> So I'm not sure that the whole "don't discard partial pages" thing is
> well-founded and I see no reason why we cannot alter it.
> 
> So, thinking caps on: why not just discard them?  After all, that's
> what userspace asked us to do.

Hi Andrew, I doubt if "just discard them" is a proper action to match 
the userspace's expectation. Maybe we will never meet the userspace's 
expectation since we are doing pages in kernel while userspace is 
passing bytes offset/length to the kernel. Note that Mel Gorman has 
already documented page-unaligned behaviors in posix_fadvise() man 
page[1] but obviously not all people (including /me) are able to read 
the _latest_ version, so someone might still uses the syscall with page 
unaligned offset/length. The userspace might only ask for discarding 
certain *bytes*, instead of *pages*.

And I think we need to look back first why we thought "preserved is 
better than discard". If we throw the whole page, the rest part of the 
page might still be required (consider the offset and length is in the 
middle of a file) because it's untagged:

   ...|------------ PAGE --------------|...
   ...| DONTNEED |------ UNTAGGED -----|...

but the page has gone, page fault occurs and we need to reload it from 
the disk -- performance degradation happens.

Maybe that's why we would rather preserv the whole page before.

But if we don't throw the partial page at all, and if the tail partial 
page is _exactly the end of the file_, a page that advised to be NONEED 
would be left in memory. And we all know that it is safe to throw it.

So we come up with this patch -- to keep the partial page not been 
throwing away, and add a special case when the partial page is the end 
of the file, we can throw it safely. I guess it might be a better solution.

One thing I'm worrying about is that, this patch might lead to a new 
undocumented behavior, so maybe we need to document this special case in 
posix_fadvise() man page too? hmmm...

Thanks,
Caspar


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Re: [PATCH] mm/fadvise: discard partial pages iff endbyte is also eof

[Mel Gorman]

On Wed, Jan 03, 2018 at 04:17:53PM -0800, Andrew Morton wrote:
> : invalidate_mapping_pages() takes start/end, but fadvise is currently passing
> : it start/len.
> : 
> : 
> : 
> :  mm/fadvise.c |    8 ++++++--
> :  1 files changed, 6 insertions(+), 2 deletions(-)
> : 
> : diff -puN mm/fadvise.c~fadvise-fix mm/fadvise.c
> : --- 25/mm/fadvise.c~fadvise-fix	2003-08-14 18:16:12.000000000 -0700
> : +++ 25-akpm/mm/fadvise.c	2003-08-14 18:16:12.000000000 -0700
> : @@ -26,6 +26,8 @@ long sys_fadvise64(int fd, loff_t offset
> :  	struct inode *inode;
> :  	struct address_space *mapping;
> :  	struct backing_dev_info *bdi;
> : +	pgoff_t start_index;
> : +	pgoff_t end_index;
> :  	int ret = 0;
> :  
> :  	if (!file)
> : @@ -65,8 +67,10 @@ long sys_fadvise64(int fd, loff_t offset
> :  	case POSIX_FADV_DONTNEED:
> :  		if (!bdi_write_congested(mapping->backing_dev_info))
> :  			filemap_flush(mapping);
> : -		invalidate_mapping_pages(mapping, offset >> PAGE_CACHE_SHIFT,
> : -				(len >> PAGE_CACHE_SHIFT) + 1);
> : +		start_index = offset >> PAGE_CACHE_SHIFT;
> : +		end_index = (offset + len + PAGE_CACHE_SIZE - 1) >>
> : +						PAGE_CACHE_SHIFT;
> : +		invalidate_mapping_pages(mapping, start_index, end_index);
> :  		break;
> :  	default:
> :  		ret = -EINVAL;
> : 
> 
> So I'm not sure that the whole "don't discard partial pages" thing is
> well-founded and I see no reason why we cannot alter it.
> 
> So, thinking caps on: why not just discard them?  After all, that's
> what userspace asked us to do.
> 

We could, it just means that any application that accidentally discards
hot data due to an unaligned fadvise will incur more IO. We've no idea
how many, if any applications, do this.

-- 
Mel Gorman
SUSE Labs

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Re: [PATCH] mm/fadvise: discard partial pages iff endbyte is also eof

[Mel Gorman]

On Thu, Jan 04, 2018 at 02:13:43PM +0800, ??????(Caspar) wrote:
> 
> 
> On 2018/1/3 18:48, Mel Gorman wrote:
> > On Wed, Jan 03, 2018 at 02:53:43PM +0800, ??????(Caspar) wrote:
> > > 
> > > 
> > > > ?? 2017??12??23????12:16?????? <shidao.ytt@alibaba-inc.com> ??????
> > > > 
> > > > From: "shidao.ytt" <shidao.ytt@alibaba-inc.com>
> > > > 
> > > > in commit 441c228f817f7 ("mm: fadvise: document the
> > > > fadvise(FADV_DONTNEED) behaviour for partial pages") Mel Gorman
> > > > explained why partial pages should be preserved instead of discarded
> > > > when using fadvise(FADV_DONTNEED), however the actual codes to calcuate
> > > > end_index was unexpectedly wrong, the code behavior didn't match to the
> > > > statement in comments; Luckily in another commit 18aba41cbf
> > > > ("mm/fadvise.c: do not discard partial pages with POSIX_FADV_DONTNEED")
> > > > Oleg Drokin fixed this behavior
> > > > 
> > > > Here I come up with a new idea that actually we can still discard the
> > > > last parital page iff the page-unaligned endbyte is also the end of
> > > > file, since no one else will use the rest of the page and it should be
> > > > safe enough to discard.
> > > 
> > > +akpm...
> > > 
> > > Hi Mel, Andrew:
> > > 
> > > Would you please take a look at this patch, to see if this proposal
> > > is reasonable enough, thanks in advance!
> > > 
> > 
> > I'm backlogged after being out for the Christmas. Superficially the patch
> > looks ok but I wondered how often it happened in practice as we already
> > would discard files smaller than a page on DONTNEED. It also requires
> 
> Actually, we would *not*. Let's look into the codes.
> 

You're right of course. I suggest updating the changelog with what you
found and the test case. I think it's reasonable to special case the
discarding of partial pages if it's the end of a file with the potential
addendum of checking if the endbyte is past the end of the file. The man
page should also be updated.

-- 
Mel Gorman
SUSE Labs

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Re: [PATCH] mm/fadvise: discard partial pages iff endbyte is also eof

[夷则(Caspar)]

On 2018/1/4 19:34, Mel Gorman wrote:
> On Thu, Jan 04, 2018 at 02:13:43PM +0800, ??????(Caspar) wrote:
>>
>>
>> On 2018/1/3 18:48, Mel Gorman wrote:
>>> On Wed, Jan 03, 2018 at 02:53:43PM +0800, ??????(Caspar) wrote:
>>>>
>>>>
>>>>> ?? 2017??12??23????12:16?????? <shidao.ytt@alibaba-inc.com> ??????
>>>>>
>>>>> From: "shidao.ytt" <shidao.ytt@alibaba-inc.com>
>>>>>
>>>>> in commit 441c228f817f7 ("mm: fadvise: document the
>>>>> fadvise(FADV_DONTNEED) behaviour for partial pages") Mel Gorman
>>>>> explained why partial pages should be preserved instead of discarded
>>>>> when using fadvise(FADV_DONTNEED), however the actual codes to calcuate
>>>>> end_index was unexpectedly wrong, the code behavior didn't match to the
>>>>> statement in comments; Luckily in another commit 18aba41cbf
>>>>> ("mm/fadvise.c: do not discard partial pages with POSIX_FADV_DONTNEED")
>>>>> Oleg Drokin fixed this behavior
>>>>>
>>>>> Here I come up with a new idea that actually we can still discard the
>>>>> last parital page iff the page-unaligned endbyte is also the end of
>>>>> file, since no one else will use the rest of the page and it should be
>>>>> safe enough to discard.
>>>>
>>>> +akpm...
>>>>
>>>> Hi Mel, Andrew:
>>>>
>>>> Would you please take a look at this patch, to see if this proposal
>>>> is reasonable enough, thanks in advance!
>>>>
>>>
>>> I'm backlogged after being out for the Christmas. Superficially the patch
>>> looks ok but I wondered how often it happened in practice as we already
>>> would discard files smaller than a page on DONTNEED. It also requires
>>
>> Actually, we would *not*. Let's look into the codes.
>>
> 
> You're right of course. I suggest updating the changelog with what you
> found and the test case. I think it's reasonable to special case the
> discarding of partial pages if it's the end of a file with the potential
> addendum of checking if the endbyte is past the end of the file. The man
> page should also be updated.

Sure, will do and send out v2.

Thanks,
Caspar
> 

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Re: [PATCH] mm/fadvise: discard partial pages iff endbyte is also eof

[Andrew Morton]

On Thu, 04 Jan 2018 16:17:50 +0800 "a?.a??(Caspar)" <jinli.zjl@alibaba-inc.com> wrote:

> > So, thinking caps on: why not just discard them?  After all, that's
> > what userspace asked us to do.
> 
> Hi Andrew, I doubt if "just discard them" is a proper action to match 
> the userspace's expectation. Maybe we will never meet the userspace's 
> expectation since we are doing pages in kernel while userspace is 
> passing bytes offset/length to the kernel. Note that Mel Gorman has 
> already documented page-unaligned behaviors in posix_fadvise() man 
> page[1] but obviously not all people (including /me) are able to read 
> the _latest_ version, so someone might still uses the syscall with page 
> unaligned offset/length. The userspace might only ask for discarding 
> certain *bytes*, instead of *pages*.
> 
> And I think we need to look back first why we thought "preserved is 
> better than discard". If we throw the whole page, the rest part of the 
> page might still be required (consider the offset and length is in the 
> middle of a file) because it's untagged:
> 
>    ...|------------ PAGE --------------|...
>    ...| DONTNEED |------ UNTAGGED -----|...
> 
> but the page has gone, page fault occurs and we need to reload it from 
> the disk -- performance degradation happens.
> 
> Maybe that's why we would rather preserv the whole page before.
> 
> But if we don't throw the partial page at all, and if the tail partial 
> page is _exactly the end of the file_, a page that advised to be NONEED 
> would be left in memory. And we all know that it is safe to throw it.
> 
> So we come up with this patch -- to keep the partial page not been 
> throwing away, and add a special case when the partial page is the end 
> of the file, we can throw it safely. I guess it might be a better solution.

OK, that makes sense.

As Mel (sort of) said, "delete part of page" can mean "I want to retain
the other part of the page".  So we should retain the page.  But for
end-of-file, there is no "other part of the page".

> One thing I'm worrying about is that, this patch might lead to a new 
> undocumented behavior, so maybe we need to document this special case in 
> posix_fadvise() man page too? hmmm...

That wouldn't hurt.

Could you please resend the patch with the changelog updated to reflect
this discussion?

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[PATCH v2] mm/fadvise: discard partial page if endbyte is also EOF

[夷则(Caspar)]

From: shidao.ytt <shidao.ytt@alibaba-inc.com>

During our recent testing with fadvise(FADV_DONTNEED), we find that if
given offset/length is not page-aligned, the last page will not be
discarded. The tool we use is vmtouch (https://hoytech.com/vmtouch/), we
map a 10KB-sized file into memory and then try to run this tool to evict
the whole file mapping, but the last single page always remains staying
in the memory:

$./vmtouch -e test_10K
           Files: 1
     Directories: 0
   Evicted Pages: 3 (12K)
         Elapsed: 2.1e-05 seconds

$./vmtouch test_10K
           Files: 1
     Directories: 0
  Resident Pages: 1/3  4K/12K  33.3%
         Elapsed: 5.5e-05 seconds

However when we test with an older kernel, say 3.10, this problem is
gone. So we wonder if this is a regression:

$./vmtouch -e test_10K
           Files: 1
     Directories: 0
   Evicted Pages: 3 (12K)
         Elapsed: 8.2e-05 seconds

$./vmtouch test_10K
           Files: 1
     Directories: 0
  Resident Pages: 0/3  0/12K  0%  <-- partial page also discarded
         Elapsed: 5e-05 seconds

After digging a little bit into this problem, we find it seems not a
regression. Not discarding partial page is likely to be on purpose
according to commit 441c228f817f7 ("mm: fadvise: document the
fadvise(FADV_DONTNEED) behaviour for partial pages") written by
Mel Gorman. He explained why partial pages should be preserved instead
of being discarded when using fadvise(FADV_DONTNEED). However, the
interesting part is that the actual code did NOT work as the same as it
was described, the partial page was still discarded anyway, due to a
calculation mistake of `end_index' passed to invalidate_mapping_pages().
This mistake has not been fixed until recently, that's why we fail to
reproduce our problem in old kernels. The fix is done in commit
18aba41cbf ("mm/fadvise.c: do not discard partial pages with
POSIX_FADV_DONTNEED") by Oleg Drokin.

Back to the original testing, our problem becomes that there is a
speical case that, if the page-unaligned `endbyte' is also the end
of file, it is not necessary at all to preserve the last partial page,
as we all know no one else will use the rest of it. It should be safe
enough if we just discard the whole page. So we add an EOF check in this
patch.

We also find a poosbile real world issue in mainline kernel. Assume such
scenario: A userspace backup application want to backup a huge amount of
small files (<4k) at once, the developer might (I guess) want to use
fadvise(FADV_DONTNEED) to save memory. However, FADV_DONTNEED won't
really happen since the only page mapped is a partial page, and kernel
will preserve it. Our patch also fixes this problem, since we know the
endbyte is EOF, so we discard it.

Here is a simple reproducer to reproduce and verify each scenario we
described above:

  test_fadvise.c
  ==============================
  #include <sys/mman.h>
  #include <sys/stat.h>
  #include <fcntl.h>
  #include <stdlib.h>
  #include <string.h>
  #include <stdio.h>
  #include <unistd.h>

  int main(int argc, char **argv)
  {
  	int i, fd, ret, len;
  	struct stat buf;
  	void *addr;
  	unsigned char *vec;
  	char *strbuf;
  	ssize_t pagesize = getpagesize();
  	ssize_t filesize;

  	fd = open(argv[1], O_RDWR|O_CREAT, S_IRUSR|S_IWUSR);
  	if (fd < 0)
  		return -1;
  	filesize = strtoul(argv[2], NULL, 10);

  	strbuf = malloc(filesize);
  	memset(strbuf, 42, filesize);
  	write(fd, strbuf, filesize);
  	free(strbuf);
  	fsync(fd);

  	len = (filesize + pagesize - 1) / pagesize;
  	printf("length of pages: %d\n", len);

  	addr = mmap(NULL, filesize, PROT_READ, MAP_SHARED, fd, 0);
  	if (addr == MAP_FAILED)
  		return -1;

  	ret = posix_fadvise(fd, 0, filesize, POSIX_FADV_DONTNEED);
  	if (ret < 0)
  		return -1;

  	vec = malloc(len);
  	ret = mincore(addr, filesize, (void *)vec);
  	if (ret < 0)
  		return -1;

  	for (i = 0; i < len; i++)
  		printf("pages[%d]: %x\n", i, vec[i] & 0x1);

  	free(vec);
  	close(fd);

  	return 0;
  }
  ==============================

Test 1: running on kernel with commit 18aba41cbf reverted:

[root@caspar ~]# uname -r
4.15.0-rc6.revert+
[root@caspar ~]# ./test_fadvise file1 1024
length of pages: 1
pages[0]: 0    # <-- partial page discarded
[root@caspar ~]# ./test_fadvise file2 8192
length of pages: 2
pages[0]: 0
pages[1]: 0
[root@caspar ~]# ./test_fadvise file3 10240
length of pages: 3
pages[0]: 0
pages[1]: 0
pages[2]: 0    # <-- partial page discarded

Test 2: running on mainline kernel:

[root@caspar ~]# uname -r
4.15.0-rc6+
[root@caspar ~]# ./test_fadvise test1 1024
length of pages: 1
pages[0]: 1    # <-- partial and the only page not discarded
[root@caspar ~]# ./test_fadvise test2 8192
length of pages: 2
pages[0]: 0
pages[1]: 0
[root@caspar ~]# ./test_fadvise test3 10240
length of pages: 3
pages[0]: 0
pages[1]: 0
pages[2]: 1    # <-- partial page not discarded

Test 3: running on kernel with this patch:

[root@caspar ~]# uname -r
4.15.0-rc6.patched+
[root@caspar ~]# ./test_fadvise test1 1024
length of pages: 1
pages[0]: 0    # <-- partial page and EOF, discarded
[root@caspar ~]# ./test_fadvise test2 8192
length of pages: 2
pages[0]: 0
pages[1]: 0
[root@caspar ~]# ./test_fadvise test3 10240
length of pages: 3
pages[0]: 0
pages[1]: 0
pages[2]: 0    # <-- partial page and EOF, discarded

Signed-off-by: shidao.ytt <shidao.ytt@alibaba-inc.com>
Signed-off-by: Caspar Zhang <jinli.zjl@alibaba-inc.com>
Reviewed-by: Oliver Yang <zhiche.yy@alibaba-inc.com>
---
v1->v2: added comments, added testcase and discussion backgrounds to
        commit msg.

 mm/fadvise.c | 11 ++++++++++-
 1 file changed, 10 insertions(+), 1 deletion(-)

diff --git a/mm/fadvise.c b/mm/fadvise.c
index ec70d6e4b86d..de00da7c03cb 100644
--- a/mm/fadvise.c
+++ b/mm/fadvise.c
@@ -127,7 +127,16 @@ SYSCALL_DEFINE4(fadvise64_64, int, fd, loff_t, offset, loff_t, len, int, advice)
 		 */
 		start_index = (offset+(PAGE_SIZE-1)) >> PAGE_SHIFT;
 		end_index = (endbyte >> PAGE_SHIFT);
-		if ((endbyte & ~PAGE_MASK) != ~PAGE_MASK) {
+		/*
+		 * page at end_index will be inclusively discarded according
+		 * to invalidate_mapping_pages() implementation, thus, minus
+		 * end_index by 1 means we would skip the last page.
+		 * Yet, if endbyte is page-aligned, or it is at the end of
+		 * file, we should not skip, discarding the last page is just
+		 * safe enough.
+		 */
+		if ((endbyte & ~PAGE_MASK) != ~PAGE_MASK &&
+				endbyte != inode->i_size - 1) {
 			/* First page is tricky as 0 - 1 = -1, but pgoff_t
 			 * is unsigned, so the end_index >= start_index
 			 * check below would be true and we'll discard the whole
-- 
2.15.1

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