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From: Nick Piggin on 25 May 2010 12:10
Hi,

I couldn't find where this patch (49bbd815fd8) was discussed, so I'll
make my own thread. Adding a few lists to cc because it might be of
interest to driver and filesystem writers.

The old ->page_mkwrite calling convention was causing problems exactly
because of this race, and we solved it by allowing page_mkwrite to
return with the page locked, and the lock will be held until the
pte is marked dirty. See commit b827e496c893de0c0f142abfaeb8730a2fd6b37f.

I hope that should provide a more elegant solution to your problem. I
would really like you to take a look at that, because we already have
filesystem code (NFS) relying on it, and more code we have relying on
this synchronization, the more chance we would find a subtle problem
with it (also it should be just nicer).

Thanks,
Nick

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From: Albert Herranz on 25 May 2010 18:20
On 05/25/2010 08:47 PM, Nick Piggin wrote:
>>> I hope that should provide a more elegant solution to your problem. I
>>> would really like you to take a look at that, because we already have
>>> filesystem code (NFS) relying on it, and more code we have relying on
>>> this synchronization, the more chance we would find a subtle problem
>>> with it (also it should be just nicer).
>>
>> So if I undestand it correctly, using the "new" calling convention I should just lock the page on fb_deferred_io_mkwrite() and return VM_FAULT_LOCKED to fix the described race for fb_defio.
>
> As far as I can see from quick reading of the fb_defio code, yes
> that should solve it (provided you lock the page inside the mutex,
> of course).
>

Ok, thanks. I'm posting a new version as RFT.

Cheers,
Albert

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