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Re: [PATCH 1/6] xfs: mmap write/read leaves bad state on pages

To: Christoph Hellwig <hch@xxxxxxxxxxxxx>
Subject: Re: [PATCH 1/6] xfs: mmap write/read leaves bad state on pages
From: Anton Altaparmakov <aia21@xxxxxxxxx>
Date: Thu, 21 Aug 2014 14:54:30 +0100
Cc: Dave Chinner <david@xxxxxxxxxxxxx>, xfs@xxxxxxxxxxx, clm@xxxxxx, linux-fsdevel@xxxxxxxxxxxxxxx
Delivered-to: xfs@xxxxxxxxxxx
In-reply-to: <20140821130812.GC1567@xxxxxxxxxxxxx>
References: <1408597754-13526-1-git-send-email-david@xxxxxxxxxxxxx> <1408597754-13526-2-git-send-email-david@xxxxxxxxxxxxx> <20140821130812.GC1567@xxxxxxxxxxxxx>
Hi,

On 21 Aug 2014, at 14:08, Christoph Hellwig <hch@xxxxxxxxxxxxx> wrote:
> On Thu, Aug 21, 2014 at 03:09:09PM +1000, Dave Chinner wrote:
>> From: Dave Chinner <dchinner@xxxxxxxxxx>
>> 
>> generic/263 is failing fsx at this point with a page spanning
>> EOF that cannot be invalidated. The operations are:
>> 
>> 1190 mapwrite   0x52c00 thru    0x5e569 (0xb96a bytes)
>> 1191 mapread    0x5c000 thru    0x5d636 (0x1637 bytes)
>> 1192 write      0x5b600 thru    0x771ff (0x1bc00 bytes)
>> 
>> where 1190 extents EOF from 0x54000 to 0x5e569. When the direct IO
>> write attempts to invalidate the cached page over this range, it
>> fails with -EBUSY and so we fire this assert:
>> 
>> XFS: Assertion failed: ret < 0 || ret == count, file: fs/xfs/xfs_file.c, 
>> line: 676
>> 
>> because the kernel is trying to fall back to buffered IO on the
>> direct IO path (which XFS does not do).
>> 
>> The real question is this: Why can't that page be invalidated after
>> it has been written to disk an cleaned?
>> 
>> Well, there's data on the first two buffers in the page (1k block
>> size, 4k page), but the third buffer on the page (i.e. beyond EOF)
>> is failing drop_buffers because it's bh->b_state == 0x3, which is
>> BH_Uptodate | BH_Dirty.  IOWs, there's dirty buffers beyond EOF. Say
>> what?
>> 
>> OK, set_buffer_dirty() is called on all buffers from
>> __set_page_buffers_dirty(), regardless of whether the buffer is
>> beyond EOF or not, which means that when we get to ->writepage,
>> we have buffers marked dirty beyond EOF that we need to clean.
>> So, we need to implement our own .set_page_dirty method that
>> doesn't dirty buffers beyond EOF.
> 
> Shouldn't this be fixed in __set_page_buffers_dirty itself?  This
> doesn't seem an XFS-specific issue.

It is perfectly normal to have dirty buffers beyond EOF.  The file system 
should just clean such buffers so the page then becomes reclaimable (or if the 
file system is using generic functionality this would be done by the generic 
functions but XFS is implementing writepage(s) itself and missing this bit).

The generic function you want to see is fs/buffer.c::__block_write_full_page() 
which does:

                if (block > last_block) {
                        /*
                         * mapped buffers outside i_size will occur, because
                         * this page can be outside i_size when there is a
                         * truncate in progress.
                         */
                        /*
                         * The buffer was zeroed by block_write_full_page()
                         */
                        clear_buffer_dirty(bh);
                        set_buffer_uptodate(bh);

Why do you want to add complexity to __set_page_buffers_dirty() given that they 
are no big deal and we need to be able to cope with their existance due to 
concurrent truncate anyway?

Best regards,

        Anton
-- 
Anton Altaparmakov <aia21 at cam.ac.uk> (replace at with @)
University of Cambridge Information Services, Roger Needham Building
7 JJ Thomson Avenue, Cambridge, CB3 0RB, UK

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