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From: David Howells <dhowells@redhat.com>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: dhowells@redhat.com, Josef Bacik <josef@toxicpanda.com>,
	Matthew Wilcox <willy@infradead.org>, Chris Mason <clm@fb.com>,
	David Sterba <dsterba@suse.com>,
	Trond Myklebust <trondmy@hammerspace.com>,
	Anna Schumaker <anna.schumaker@netapp.com>,
	Steve French <sfrench@samba.org>,
	Dominique Martinet <asmadeus@codewreck.org>,
	Christoph Hellwig <hch@lst.de>,
	Alexander Viro <viro@zeniv.linux.org.uk>,
	Linux-MM <linux-mm@kvack.org>,
	linux-cachefs@redhat.com, linux-afs@lists.infradead.org,
	"open list:NFS, SUNRPC, AND..." <linux-nfs@vger.kernel.org>,
	CIFS <linux-cifs@vger.kernel.org>,
	ceph-devel@vger.kernel.org, v9fs-developer@lists.sourceforge.net,
	linux-fsdevel <linux-fsdevel@vger.kernel.org>,
	Jeff Layton <jlayton@redhat.com>,
	David Wysochanski <dwysocha@redhat.com>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH v4 02/28] mm: Add an unlock function for PG_private_2/PG_fscache
Date: Wed, 17 Mar 2021 09:04:09 +0000	[thread overview]
Message-ID: <31382.1615971849@warthog.procyon.org.uk> (raw)
In-Reply-To: <CAHk-=whWoJhGeMn85LOh9FX-5d2-Upzmv1m2ZmYxvD31TKpUTA@mail.gmail.com>

Linus Torvalds <torvalds@linux-foundation.org> wrote:

> And as far as I can tell, fscache doesn't want that PG_private_2 bit
> to interact with the random VM lifetime or migration rules either, and
> should rely entirely on the page count. David?

It's slightly complicated for fscache as there are two separate pieces of code
involved:

 (1) For the old fscache code that I'm trying to phase out, it does not take a
     ref when PG_fscache is taken (probably incorrectly), relying instead on
     releasepage, etc. getting called to strip the PG_fscache bit.  PG_fscache
     is held for the lifetime of the page, indicating that fscache knows about
     it and might access it at any time (to write to the cache in the
     background for example or to move pages around in the cache).

     Here PG_fscache should not prevent page eviction or migration and it's
     analogous to PG_private.

     That said, the old fscache code keeps its own radix trees of pages that
     are undergoing write to the cache, so to allow a page to be evicted,
     releasepage and co. have to consult those
     (__fscache_maybe_release_page()).

 (2) For the new netfs lib, PG_fscache is ignored by fscache itself and is
     used by the read helpers.  The helpers simply use it analogously to
     PG_writeback, indicating that there's I/O in progress from this page to
     the cache[*].  It's fine to take a ref here because we know we'll drop it
     shortly.

     Here PG_fscache might prevent page eviction or migration, but only
     because I/O is in progress.  If an increment on the page refcount
     suffices, that's fine.

In both cases, releasepage, etc. look at PG_fscache and decide whether to wait
or not (releasepage may tell the caller to skip the page if PG_fscache is
set).

[*] Willy suggested using PG_writeback to cover both write to the server and
write to the cache, and getting rid of PG_fscache entirely, but that would
require extra mechanisms.  There are three cases:

 (a) We might be writing to only the cache, e.g. because we just read from the
     server.

     Note that this may adversely affect code that does accounting associated
     with PG_writeback because we woudn't actually be writing back a user-made
     change or dealing with a dirty page.  I'm not sure if that's an issue.

 (b) We might writing to both, in which case we can expect both writes to
     finish at different times.

 (c) We might only be writing to the server, e.g. because there's no space in
     the cache or there is no cache.

It's something that might make sense, however, and we can look at in the
future, but for the moment having two separate page flags is simplest.

An additional use of PG_fscache is to prevent a second write to the cache from
being started whilst one is in progress.  I guess that would be taken over by
PG_writeback if we used that.

David


  parent reply	other threads:[~2021-03-17  9:05 UTC|newest]

Thread overview: 49+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-03-10 16:54 [PATCH v4 00/28] Network fs helper library & fscache kiocb API David Howells
2021-03-10 16:54 ` [PATCH v4 01/28] iov_iter: Add ITER_XARRAY David Howells
2021-03-10 16:54 ` [PATCH v4 02/28] mm: Add an unlock function for PG_private_2/PG_fscache David Howells
2021-03-16 19:07   ` Matthew Wilcox
2021-03-17  0:43     ` Linus Torvalds
2021-03-17  0:43       ` Linus Torvalds
2021-03-17  2:12       ` Josef Bacik
2021-03-17  2:35         ` Linus Torvalds
2021-03-17  2:35           ` Linus Torvalds
2021-03-17  9:04         ` David Howells [this message]
2021-03-17 10:53         ` David Howells
2021-03-16 20:38   ` David Howells
2021-03-16 23:32     ` Matthew Wilcox
2021-03-21 10:53   ` Matthew Wilcox
2021-03-22 10:56   ` David Howells
2021-03-23 13:17   ` David Howells
2021-03-23 13:51     ` Matthew Wilcox
2021-03-23 14:16     ` David Howells
2021-03-23 22:06     ` David Howells
2021-03-23 22:06       ` David Howells
2021-03-10 16:55 ` [PATCH v4 03/28] mm: Implement readahead_control pageset expansion David Howells
2021-03-10 16:55 ` [PATCH v4 04/28] netfs: Make a netfs helper module David Howells
2021-03-10 16:55 ` [PATCH v4 05/28] netfs: Documentation for helper library David Howells
2021-03-10 16:55 ` [PATCH v4 06/28] netfs, mm: Move PG_fscache helper funcs to linux/netfs.h David Howells
2021-03-10 16:56 ` [PATCH v4 07/28] netfs, mm: Add unlock_page_fscache() and wait_on_page_fscache() David Howells
2021-03-10 16:56 ` [PATCH v4 08/28] netfs: Provide readahead and readpage netfs helpers David Howells
2021-03-21  1:42   ` Matthew Wilcox
2021-03-22 17:13   ` David Howells
2021-03-10 16:56 ` [PATCH v4 09/28] netfs: Add tracepoints David Howells
2021-03-10 16:56 ` [PATCH v4 10/28] netfs: Gather stats David Howells
2021-03-10 16:56 ` [PATCH v4 11/28] netfs: Add write_begin helper David Howells
2021-03-10 16:57 ` [PATCH v4 12/28] netfs: Define an interface to talk to a cache David Howells
2021-03-10 16:57 ` [PATCH v4 13/28] netfs: Hold a ref on a page when PG_private_2 is set David Howells
2021-03-10 16:57 ` [PATCH v4 14/28] fscache, cachefiles: Add alternate API to use kiocb for read/write to cache David Howells
2021-03-10 16:57 ` [PATCH v4 15/28] afs: Disable use of the fscache I/O routines David Howells
2021-03-10 16:58 ` [PATCH v4 16/28] afs: Pass page into dirty region helpers to provide THP size David Howells
2021-03-10 16:58 ` [PATCH v4 17/28] afs: Print the operation debug_id when logging an unexpected data version David Howells
2021-03-10 16:58 ` [PATCH v4 18/28] afs: Move key to afs_read struct David Howells
2021-03-10 16:58 ` [PATCH v4 19/28] afs: Don't truncate iter during data fetch David Howells
2021-03-10 16:58 ` [PATCH v4 20/28] afs: Log remote unmarshalling errors David Howells
2021-03-10 16:59 ` [PATCH v4 21/28] afs: Set up the iov_iter before calling afs_extract_data() David Howells
2021-03-10 16:59 ` [PATCH v4 22/28] afs: Use ITER_XARRAY for writing David Howells
2021-03-10 16:59 ` [PATCH v4 23/28] afs: Wait on PG_fscache before modifying/releasing a page David Howells
2021-03-10 16:59 ` [PATCH v4 24/28] afs: Extract writeback extension into its own function David Howells
2021-03-10 16:59 ` [PATCH v4 25/28] afs: Prepare for use of THPs David Howells
2021-03-10 17:00 ` [PATCH v4 26/28] afs: Use the fs operation ops to handle FetchData completion David Howells
2021-03-10 17:00 ` [PATCH v4 27/28] afs: Use new fscache read helper API David Howells
2021-03-10 17:00 ` [PATCH v4 28/28] afs: Use the fscache_write_begin() helper David Howells
2021-03-16 11:34 ` [PATCH v4 08/28] netfs: Provide readahead and readpage netfs helpers David Howells

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