On 10/03/2013 08:09 PM, Miklos Szeredi wrote:
On Thu, Oct 3, 2013 at 5:50 PM, Maxim Patlasov <mpatlasov@xxxxxxxxxxxxx> wrote:
On 10/03/2013 07:14 PM, Miklos Szeredi wrote:
On Thu, Oct 03, 2013 at 05:28:30PM +0400, Maxim Patlasov wrote:
1. There is an in-flight primary request with a chain of secondary ones.
2. User calls ftruncate(2) to extend file; fuse_set_nowrite() makes
fi->writectr negative and starts waiting for completion of that
in-flight request
3. Userspace fuse daemon ACKs the request and fuse_writepage_end()
is called; it calls __fuse_flush_writepages(), but the latter does
nothing because fi->writectr < 0
4. fuse_do_setattr() proceeds extending i_size and calling
__fuse_release_nowrite(). But now new (increased) i_size will be
used as 'crop' arg of __fuse_flush_writepages()
stale data can leak to the server.
So, lets do this then: skip fuse_flush_writepages() and call
fuse_send_writepage() directly. It will ignore the NOWRITE logic, but
that's
okay, this happens rarely and cannot happen more than once in a row.
Does this look good?
Yes, but let's at least add a comment explaining why it's safe. There are
three different cases and what you write above explains only one of them:
1st case (trivial): there are no concurrent activities using
fuse_set/release_nowrite. Then we're on safe side because
fuse_flush_writepages() would call fuse_send_writepage() anyway.
2nd case: someone called fuse_set_nowrite and it is waiting now for
completion of all in-flight requests. Here what you wrote about "happening
rarely and no more than once" is applicable.
3rd case: someone (e.g. fuse_do_setattr()) is in the middle of
fuse_set_nowrite..fuse_release_nowrite section. The fact that
fuse_set_nowrite returned implies that all in-flight requests were completed
along with all its secondary requests (because we increment writectr for a
secondry before decrementing it for the primary -- that's how
fuse_writepage_end is implemeted). Further requests are blocked by negative
writectr. Hence there cannot be any in-flight requests and no invocations of
fuse_writepage_end while we're in fuse_set_nowrite..fuse_release_nowrite
section.
It looks obvious now, but I'm not sure we'll able to recollect it later.
Added your analysis as a comment and all patches pushed to writepages.v2.
Great! So I can proceed with re-basing the rest of
writeback-cache-policy pile to writepages.v2 soon.
Can you actually trigger this path with your testing?
No.
Hmm, did you do any testing with the wait-for-page-writeback disabled
in fuse_mkwrite()?
Yes, of course. I've been doing that for a week on two very different
h/w nodes, but I'm using ordinary fsx (not some artificial hand-crafted
mmap-writer) and I usually get only a dozen "rewrite: 1" messages per
night. This is enough to make sure that rewrite code main-path is OK,
but not enough to be sure that all corner cases are covered.
Thanks,
Maxim
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