| Re: [RFC][PATCH] EXT3: problem with page fault inside a transaction [message #7361] |
Thu, 12 October 2006 06:43  |
Andrew Morton
Messages: 127
Registered: December 2005
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Senior Member |
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On Thu, 12 Oct 2006 09:57:26 +0400
Dmitriy Monakhov <dmonakhov@openvz.org> wrote:
> While reading Andrew's generic_file_buffered_write patches i've remembered
> one more EXT3 issue.journal_start() in prepare_write() causes different ranking
> violations if copy_from_user() triggers a page fault. It could cause
> GFP_FS allocation, re-entering into ext3 code possibly with a different
> superblock and journal, ranking violation of journalling serialization
> and mmap_sem and page lock and all other kinds of funny consequences.
With the stuff Nick and I are looking at, we won't take pagefaults inside
prepare_write()/commit_write() any more.
> Our customers complain about this issue.
Really? How often?
What on earth are they doing to trigger this? writev() without the 2.6.18
writev() bugfix?
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| Re: [RFC][PATCH] EXT3: problem with page fault inside a transaction [message #7372 is a reply to message #7367] |
Thu, 12 October 2006 08:31  |
Nick Piggin
Messages: 35
Registered: March 2006
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Member |
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Dmitriy Monakhov wrote:
> Andrew Morton <akpm@osdl.org> writes:
>>With the stuff Nick and I are looking at, we won't take pagefaults inside
>>prepare_write()/commit_write() any more.
>
> I'sorry may be i've missed something, but how cant you prevent this?
>
> Let's look at generic_file_buffered_write:
> #### force page fault
> fault_in_pages_readable();
>
> ### find and lock page
> __grab_cache_page()
>
> #### allocate blocks.This may result in low memory condition
> #### try_to_free_pages->shrink_caches() and etc.
> a_ops->prepare_write()
>
> ### can anyone guarantee that page fault hasn't happened by now ?
Yes. Do an atomic copy, which will early exit from the pagefault handler
and return a short copy. Then close up the write, drop the page lock,
and rerun the fault_in_pages_readable, which will do the full pagefaults
for us, then try again.
Regardless of what you do to ext3, the VM just can't handle a fault
here anyway.
--
SUSE Labs, Novell Inc.
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