[Linux-cachefs] [PATCH 5/5] ceph: fold ceph_update_writeable_page into ceph_write_begin

[Andrew W Elble]

David Howells <dhowells at redhat.com> writes:

> Matthew Wilcox <willy at infradead.org> wrote:
>
>> Yes.  I do that kind of thing in iomap.  What you're doing there looks
>> a bit like offset_in_folio(), but I don't understand the problem with
>> just checking pos against i_size directly.
>
> pos can be in the middle of a page.  If i_size is at, say, the same point in
> the middle of that page and the page isn't currently in the cache, then we'll
> just clear the entire page and not read the bottom part of it (ie. the bit
> prior to the EOF).
>
> It's odd, though, that xfstests doesn't catch this.

Some more details/information:

1.) The test is a really simple piece of code that essentially writes
    a incrementing number to a file on 'node a', while a 'tail -f'
    is run and exited multiple times on 'node b'. We haven't had much
    luck in this causing the problem when done on a single node
2.) ((pos & PAGE_MASK) >= i_size_read(inode)) ||' merely reflects the
    logic that was in place prior to 1cc1699070bd. Patching
    fs/ceph/addr.c with this does seem to eliminate the corruption.
    (We'll test with the patch Jeff posted here shortly)
3.) After finding all of this, I went upstream to look at fs/ceph/addr.c
    and discovered the move to leveraging fs/netfs/read_helper.c - we've
    only just compiled against git master and replicated the corruption
    there.


-- 
Andrew W. Elble
Infrastructure Engineer
Information and Technology Services
Rochester Institute of Technology
tel: (585)-475-2411 | aweits at rit.edu
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