Re: [RFC][PATCH] EXT3: problem with page fault inside a transaction

From: Nick Piggin
Date: Thu Oct 12 2006 - 04:31:46 EST


Dmitriy Monakhov wrote:
Andrew Morton <akpm@xxxxxxxx> 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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