page_mkwrite vs pte dirty race in fb_defio

page_mkwrite vs pte dirty race in fb_defio

[Nick Piggin]

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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Re: page_mkwrite vs pte dirty race in fb_defio

[Albert Herranz]

Hi,

On 05/25/2010 06:01 PM, Nick Piggin wrote:
> 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 original thread can be found here:
http://marc.info/?l=linux-fbdev&m=127369791432181

> 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.
> 

Ah, didn't know about that. Thanks for the pointer.

> 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.

> Thanks,
> Nick
> 

Thanks,
Albert

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Re: page_mkwrite vs pte dirty race in fb_defio

[Nick Piggin]

On Tue, May 25, 2010 at 08:26:31PM +0200, Albert Herranz wrote:
> Hi,
> 
> On 05/25/2010 06:01 PM, Nick Piggin wrote:
> > 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 original thread can be found here:
> http://marc.info/?l=linux-fbdev&m=127369791432181

Thanks.

 
> > 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.
> > 
> 
> Ah, didn't know about that. Thanks for the pointer.
> 
> > 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).

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Re: [Bulk] Re: page_mkwrite vs pte dirty race in fb_defio

[Albert Herranz]

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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