From: Maxim Patlasov <mpatlasov@parallels.com>
To: Miklos Szeredi <miklos@szeredi.hu>
Cc: <riel@redhat.com>, Kirill Korotaev <dev@parallels.com>,
Pavel Emelianov <xemul@parallels.com>,
fuse-devel <fuse-devel@lists.sourceforge.net>,
Brian Foster <bfoster@redhat.com>,
Kernel Mailing List <linux-kernel@vger.kernel.org>,
James Bottomley <jbottomley@parallels.com>, <linux-mm@kvack.org>,
Al Viro <viro@zeniv.linux.org.uk>,
Linux-Fsdevel <linux-fsdevel@vger.kernel.org>,
Andrew Morton <akpm@linux-foundation.org>,
<fengguang.wu@intel.com>, <devel@openvz.org>,
Mel Gorman <mgorman@suse.de>
Subject: Re: [PATCH 10/16] fuse: Implement writepages callback
Date: Fri, 9 Aug 2013 19:02:12 +0400 [thread overview]
Message-ID: <52050474.8040608@parallels.com> (raw)
In-Reply-To: <CAJfpegtr4+vv_ZzuM7EE7MkHPqNi4brQamg4ZOWb2Me+iG87JQ@mail.gmail.com>
Hi Miklos,
08/06/2013 08:25 PM, Miklos Szeredi пишет:
> On Fri, Aug 2, 2013 at 5:40 PM, Maxim Patlasov <mpatlasov@parallels.com> wrote:
>> 07/19/2013 08:50 PM, Miklos Szeredi пишет:
>>
>>> On Sat, Jun 29, 2013 at 09:45:29PM +0400, Maxim Patlasov wrote:
>>>> From: Pavel Emelyanov <xemul@openvz.org>
>>>>
>>>> The .writepages one is required to make each writeback request carry more
>>>> than
>>>> one page on it. The patch enables optimized behaviour unconditionally,
>>>> i.e. mmap-ed writes will benefit from the patch even if
>>>> fc->writeback_cache=0.
>>> I rewrote this a bit, so we won't have to do the thing in two passes,
>>> which
>>> makes it simpler and more robust. Waiting for page writeback here is
>>> wrong
>>> anyway, see comment above fuse_page_mkwrite(). BTW we had a race there
>>> because
>>> fuse_page_mkwrite() didn't take the page lock. I've also fixed that up
>>> and
>>> pushed a series containing these patches up to implementing ->writepages()
>>> to
>>>
>>> git://git.kernel.org/pub/scm/linux/kernel/git/mszeredi/fuse.git
>>> writepages
>>>
>>> Passed some trivial testing but more is needed.
>>
>> Thanks a lot for efforts. The approach you implemented looks promising, but
>> it introduces the following assumption: a page cannot become dirty before we
>> have a chance to wait on fuse writeback holding the page locked. This is
>> already true for mmap-ed writes (due to your fixes) and it seems doable for
>> cached writes as well (like we do in fuse_perform_write). But the assumption
>> seems to be broken in case of direct read from local fs (e.g. ext4) to a
>> memory region mmap-ed to a file on fuse fs. See how dio_bio_submit() marks
>> pages dirty by bio_set_pages_dirty(). I can't see any solution for this
>> use-case. Do you?
> Hmm. Direct IO on an mmaped file will do get_user_pages() which will
> do the necessary page fault magic and ->page_mkwrite() will be called.
> At least AFAICS.
Yes, I agree.
>
> The page cannot become dirty through a memory mapping without first
> switching the pte from read-only to read-write first. Page accounting
> logic relies on this too. The other way the page can become dirty is
> through write(2) on the fs. But we do get notified about that too.
Yes, that's correct, but I don't understand why you disregard two other
cases of marking page dirty (both related to direct AIO read from a file
to a memory region mmap-ed to a fuse file):
1. dio_bio_submit() -->
bio_set_pages_dirty() -->
set_page_dirty_lock()
2. dio_bio_complete() -->
bio_check_pages_dirty() -->
bio_dirty_fn() -->
bio_set_pages_dirty() -->
set_page_dirty_lock()
As soon as a page became dirty through a memory mapping (exactly as you
explained), nothing would prevent it to be written-back. And fuse will
call end_page_writeback almost immediately after copying the real page
to a temporary one. Then dio_bio_submit may re-dirty page speculatively
w/o notifying fuse. And again, since then nothing would prevent it to be
written-back once more. Hence we can end up in more then one temporary
page in fuse write-back. And similar concern for dio_bio_complete()
re-dirty.
This make me think that we do need fuse_page_is_writeback() in
fuse_writepages_fill(). But it shouldn't be harmful because it will
no-op practically always due to waiting for fuse writeback in
->page_mkwrite() and in course of handling write(2).
Thanks,
Maxim
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next prev parent reply other threads:[~2013-08-09 15:02 UTC|newest]
Thread overview: 35+ messages / expand[flat|nested] mbox.gz Atom feed top
2013-06-29 17:41 [PATCH v5 00/16] fuse: An attempt to implement a write-back cache policy Maxim Patlasov
2013-06-29 17:42 ` [PATCH 01/16] fuse: Linking file to inode helper Maxim Patlasov
2013-06-29 17:42 ` [PATCH 02/16] fuse: Getting file for writeback helper Maxim Patlasov
2013-06-29 17:42 ` [PATCH 04/16] fuse: Prepare to handle multiple pages in writeback Maxim Patlasov
2013-06-29 17:42 ` [PATCH 05/16] fuse: Connection bit for enabling writeback Maxim Patlasov
2013-06-29 17:44 ` [PATCH 06/16] fuse: Trust kernel i_size only - v4 Maxim Patlasov
2013-06-29 17:44 ` [PATCH 07/16] fuse: Trust kernel i_mtime only Maxim Patlasov
2013-07-11 16:14 ` [PATCH 07/16] fuse: Trust kernel i_mtime only -v2 Maxim Patlasov
2013-06-29 17:45 ` [PATCH 08/16] fuse: Flush files on wb close Maxim Patlasov
2013-07-11 16:18 ` [PATCH 08/16] fuse: Flush files on wb close -v2 Maxim Patlasov
[not found] ` <20130629172211.20175.70154.stgit-vWG5eQQidJHciZdyczg/7Q@public.gmane.org>
2013-06-29 17:42 ` [PATCH 03/16] fuse: Prepare to handle short reads Maxim Patlasov
2013-06-29 17:45 ` [PATCH 09/16] fuse: restructure fuse_readpage() Maxim Patlasov
2013-06-29 17:45 ` [PATCH 10/16] fuse: Implement writepages callback Maxim Patlasov
2013-07-19 16:50 ` Miklos Szeredi
2013-08-02 15:40 ` Maxim Patlasov
2013-08-06 16:25 ` Miklos Szeredi
2013-08-06 16:26 ` Eric Boxer
2013-08-09 15:02 ` Maxim Patlasov [this message]
2013-08-30 10:12 ` Miklos Szeredi
2013-08-30 14:50 ` Maxim Patlasov
2013-09-03 10:31 ` Miklos Szeredi
2013-09-03 16:02 ` Maxim Patlasov
2013-06-29 17:45 ` [PATCH 11/16] fuse: Implement write_begin/write_end callbacks Maxim Patlasov
2013-06-29 17:46 ` [PATCH 12/16] fuse: fuse_writepage_locked() should wait on writeback Maxim Patlasov
2013-06-29 17:46 ` [PATCH 13/16] fuse: fuse_flush() " Maxim Patlasov
2013-06-29 17:46 ` [PATCH 14/16] fuse: Fix O_DIRECT operations vs cached writeback misorder - v2 Maxim Patlasov
2013-06-29 17:47 ` [PATCH 15/16] fuse: Turn writeback cache on Maxim Patlasov
2013-06-29 17:48 ` [PATCH 16/16] mm: strictlimit feature Maxim Patlasov
2013-07-01 21:16 ` Andrew Morton
2013-07-02 8:33 ` Maxim Patlasov
2013-07-02 17:44 ` [PATCH] mm: strictlimit feature -v2 Maxim Patlasov
2013-07-02 19:38 ` Andrew Morton
2013-07-03 11:01 ` Maxim Patlasov
2013-07-03 23:16 ` Jan Kara
2013-07-05 13:14 ` Maxim Patlasov
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