Re: [NEED TESTERS] remove swapin_readahead Re: shmem_getpage_locked() / swapin_readahead() race in 2.4.4-pre3

["Stephen C. Tweedie" <sct@redhat.com>]

Hi,

On Sat, Apr 14, 2001 at 08:31:07PM -0300, Marcelo Tosatti wrote:
> On Sat, 14 Apr 2001, Rik van Riel wrote:
> > On Sat, 14 Apr 2001, Marcelo Tosatti wrote:
> > 
> > > There is a nasty race between shmem_getpage_locked() and
> > > swapin_readahead() with the new shmem code (introduced in
> > > 2.4.3-ac3 and merged in the main tree in 2.4.4-pre3):

> Test (multiple shm-stress) runs fine without swapin_readahead(), as
> expected.

> Stephen/Linus? 

I don't see the problem.  shmem_getpage_locked appears to back off
correctly if it encounters a swap-cached page already existing if
swapin_readahead has installed the page first, at least with the code
in 2.4.3-ac5.

There *does* appear to be a race, but it's swapin_readahead racing
with shmem_writepage.  That code does not search for an existing entry
in the swap cache when it decides to move a shmem page to swap, so we
can install the page twice and end up doing a lookup on the wrong
physical page if there is swap readahead going on.

To fix that, shmem_writepage needs to do a swap cache lookup and lock
before installing the new page --- it should probably just copy the
new page into the old one if it finds one already there.

--Stephen