Re: [PATCH] hugetlbfs: dirty pages as they are added to pagecache

[Mike Kravetz]

On 10/23/18 12:43 AM, Michal Hocko wrote:
> On Wed 17-10-18 21:10:22, Mike Kravetz wrote:
>> Some test systems were experiencing negative huge page reserve
>> counts and incorrect file block counts.  This was traced to
>> /proc/sys/vm/drop_caches removing clean pages from hugetlbfs
>> file pagecaches.  When non-hugetlbfs explicit code removes the
>> pages, the appropriate accounting is not performed.
>>
>> This can be recreated as follows:
>>  fallocate -l 2M /dev/hugepages/foo
>>  echo 1 > /proc/sys/vm/drop_caches
>>  fallocate -l 2M /dev/hugepages/foo
>>  grep -i huge /proc/meminfo
>>    AnonHugePages:         0 kB
>>    ShmemHugePages:        0 kB
>>    HugePages_Total:    2048
>>    HugePages_Free:     2047
>>    HugePages_Rsvd:    18446744073709551615
>>    HugePages_Surp:        0
>>    Hugepagesize:       2048 kB
>>    Hugetlb:         4194304 kB
>>  ls -lsh /dev/hugepages/foo
>>    4.0M -rw-r--r--. 1 root root 2.0M Oct 17 20:05 /dev/hugepages/foo
>>
>> To address this issue, dirty pages as they are added to pagecache.
>> This can easily be reproduced with fallocate as shown above. Read
>> faulted pages will eventually end up being marked dirty.  But there
>> is a window where they are clean and could be impacted by code such
>> as drop_caches.  So, just dirty them all as they are added to the
>> pagecache.
>>
>> In addition, it makes little sense to even try to drop hugetlbfs
>> pagecache pages, so disable calls to these filesystems in drop_caches
>> code.
>>
>> Fixes: 70c3547e36f5 ("hugetlbfs: add hugetlbfs_fallocate()")
>> Cc: stable@xxxxxxxxxxxxxxx
>> Signed-off-by: Mike Kravetz <mike.kravetz@xxxxxxxxxx>
> 
> I do agree with others that HUGETLBFS_MAGIC check in drop_pagecache_sb
> is wrong in principal. I am not even sure we want to special case memory
> backed filesystems. What if we ever implement MADV_FREE on fs? Should
> those pages be dropped? My first idea take would be yes.

Ok, I have removed that hard coded check.  Implementing MADV_FREE on
hugetlbfs would take some work, but it could be done.

> Acked-by: Michal Hocko <mhocko@xxxxxxxx> to the set_page_dirty dirty
> part.
> 
> Although I am wondering why you haven't covered only the fallocate path
> wrt Fixes tag. In other words, do we need the same treatment for the
> page fault path? We do not set dirty bit on page there as well. We rely
> on the dirty bit in pte and only for writable mappings. I have hard time
> to see why we have been safe there as well. So maybe it is your Fixes:
> tag which is not entirely correct, or I am simply missing the fault
> path.

No, you are not missing anything.  In the commit log I mentioned that this
also does apply to the fault path.  The change takes care of them both.

I was struggling with what to put in the fixes tag.  As mentioned, this
problem also exists in the fault path.  Since 3.16 is the oldest stable
release, I went back and used the commit next to the add_to_page_cache code
there.  However, that seems kind of random.  Is there a better way to say
the patch applies to all stable releases?

Here is updated patch without the drop_caches change and updated fixes tag.

From: Mike Kravetz <mike.kravetz@xxxxxxxxxx>

hugetlbfs: dirty pages as they are added to pagecache

Some test systems were experiencing negative huge page reserve
counts and incorrect file block counts.  This was traced to
/proc/sys/vm/drop_caches removing clean pages from hugetlbfs
file pagecaches.  When non-hugetlbfs explicit code removes the
pages, the appropriate accounting is not performed.

This can be recreated as follows:
 fallocate -l 2M /dev/hugepages/foo
 echo 1 > /proc/sys/vm/drop_caches
 fallocate -l 2M /dev/hugepages/foo
 grep -i huge /proc/meminfo
   AnonHugePages:         0 kB
   ShmemHugePages:        0 kB
   HugePages_Total:    2048
   HugePages_Free:     2047
   HugePages_Rsvd:    18446744073709551615
   HugePages_Surp:        0
   Hugepagesize:       2048 kB
   Hugetlb:         4194304 kB
 ls -lsh /dev/hugepages/foo
   4.0M -rw-r--r--. 1 root root 2.0M Oct 17 20:05 /dev/hugepages/foo

To address this issue, dirty pages as they are added to pagecache.
This can easily be reproduced with fallocate as shown above. Read
faulted pages will eventually end up being marked dirty.  But there
is a window where they are clean and could be impacted by code such
as drop_caches.  So, just dirty them all as they are added to the
pagecache.

Fixes: 6bda666a03f0 ("hugepages: fold find_or_alloc_pages into huge_no_page()")
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Mike Kravetz <mike.kravetz@xxxxxxxxxx>
---
 mm/hugetlb.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index 5c390f5a5207..7b5c0ad9a6bd 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -3690,6 +3690,12 @@ int huge_add_to_page_cache(struct page *page, struct address_space *mapping,
 		return err;
 	ClearPagePrivate(page);
 
+	/*
+	 * set page dirty so that it will not be removed from cache/file
+	 * by non-hugetlbfs specific code paths.
+	 */
+	set_page_dirty(page);
+
 	spin_lock(&inode->i_lock);
 	inode->i_blocks += blocks_per_huge_page(h);
 	spin_unlock(&inode->i_lock);
-- 
2.17.2