[Linux-cachefs] Files read from NFS instead of fscache when also in pagecache

[Ole Salscheider]

On 02.08.23 17:08, David Wysochanski wrote:
> On Wed, Aug 2, 2023 at 3:35 AM Ole Salscheider <ole at salscheider.org> wrote:
>>
>> Hello,
>>
>> I noticed some strange behaviour of fscache with kernel 6.4.x in
>> combination with NFS (v4.2) and the pagecache.
>>
>> I have multiple clients mounting an NFS v4.2 file system from a server
>> and caching the data locally using fscache. After booting the clients,
>> most data is served from fscache and only a few requests hit the NFS
>> server. But after some minutes of operation (which typically includes a
>> lot of cyclic data reading for us), fewer and fewer files are served
>> from fscache and most are served by the NFS server.
>> It seems that this has something to do with the files being also in the
>> pagecache. If I periodcally drop the pagecache with "echo 3 >
>> /proc/sys/vm/drop_caches", then most of the files are served from
>> fscache again.
>>
>> Is this a known problem? If not, do you have any suggestion how I can
>> try to find the root cause?
>>
> 
> Yes this is most likely a known problem with a fix now in the
> mm-unstable branch at
> git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm
> If it's the same problem, take these 3 patches and that should fix it:
> 6fbefbe174ca mm: call folio_mapping() inside folio_needs_release()
> 04cb8ddab902 mm, netfs, fscache: stop read optimisation when folio
> removed from pagecache
> 854fc8a567ab mm: merge folio_has_private()/filemap_release_folio() call pairs
> 
> The main patch that fixes it is "04cb8ddab902 mm, netfs, fscache: Stop
> read optimisation when folio removed from pagecache" but there's a
> prerequisite patch and a fixup patch.

I ran some tests yesterday and today, and it seems that these patches 
fix the problem completely. Thanks!

> 
> 
>> Thank you in advance!
>>
>> Best regards,
>>
>> Ole
>>
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>