Re: [PATCH RFC] mm: fix refcount check in mapping_evict_folio

[David Hildenbrand <david@redhat.com>]

On 20.08.24 10:00, David Hildenbrand wrote:
> On 13.08.24 20:25, Boris Burkov wrote:
>> The commit e41c81d0d30e ("mm/truncate: Replace page_mapped() call in
>> invalidate_inode_page()") replaced the page_mapped(page) check with a
>> refcount check. However, this refcount check does not work as expected
>> with drop_caches, at least for btrfs's metadata pages.
>>
>> Btrfs has a per-sb metadata inode with cached pages, and when not in
>> active use by btrfs, they have a refcount of 3. One from the initial
>> call to alloc_pages, one (nr_pages == 1) from filemap_add_folio, and one
>> from folio_attach_private. We would expect such pages to get dropped by
>> drop_caches. However, drop_caches calls into mapping_evict_folio via
>> mapping_try_invalidate which gets a reference on the folio with
>> find_lock_entries(). As a result, these pages have a refcount of 4, and
>> fail this check.
>>
>> For what it's worth, such pages do get reclaimed under memory pressure,
>> and if I change this refcount check to `if folio_mapped(folio)`
>>
>> The following script produces such pages and uses drgn to further
>> analyze the state of the folios:
>> https://github.com/boryas/scripts/blob/main/sh/strand-meta/run.sh
>> It should at least outline the basic idea for producing some btrfs
>> metadata via creating inlined-extent files.
>>
>> My proposed fix for the issue is to add one more hardcoded refcount to
>> this check to account for the caller having a refcount on the page.
>> However, I am less familiar with the other caller into
>> mapping_evict_folio in the page fault path, so I am concerned this fix
>> will not work properly there, and would appreciate extra scrutiny there.
>>
>> Fixes: e41c81d0d30e ("mm/truncate: Replace page_mapped() call in invalidate_inode_page()")
>> Signed-off-by: Boris Burkov <boris@bur.io>
>> ---
>>    mm/truncate.c | 12 ++++++++++--
>>    1 file changed, 10 insertions(+), 2 deletions(-)
>>
>> diff --git a/mm/truncate.c b/mm/truncate.c
>> index 4d61fbdd4b2f..c710c84710b4 100644
>> --- a/mm/truncate.c
>> +++ b/mm/truncate.c
>> @@ -267,9 +267,17 @@ long mapping_evict_folio(struct address_space *mapping, struct folio *folio)
>>    		return 0;
>>    	if (folio_test_dirty(folio) || folio_test_writeback(folio))
>>    		return 0;
>> -	/* The refcount will be elevated if any page in the folio is mapped */
>> +	/*
>> +	 * The refcount will be elevated if any page in the folio is mapped.
>> +	 *
>> +	 * The refcounts break down as follows:
>> +	 * 1 per mapped page
> 
> Using "mapped" is confusing -- I would have expected a folio_mapcount()
> here.
> 
> Did you mean "1 reference from the pagecache to each page"?

... and now I spotted that Willy had the same comment already, good :)

-- 
Cheers,

David / dhildenb