linux-kernel - Re: add_to_swap_cache with GFP

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Date:	Tue, 31 Mar 2009 20:30:35 +0900
From:	Minchan Kim <minchan.kim@...il.com>
To:	Hugh Dickins <hugh@...itas.com>
Cc:	linux-mm <linux-mm@...ck.org>, lkml <linux-kernel@...r.kernel.org>
Subject: Re: add_to_swap_cache with GFP_ATOMIC ?

Thanks for quick reply.

On Tue, Mar 31, 2009 at 8:17 PM, Hugh Dickins <hugh@...itas.com> wrote:
> On Tue, 31 Mar 2009, Minchan Kim wrote:
>>
>> I don't know why we should call add_to_swap_cache with GFP_ATOMIC ?
>> Is there a special something for avoiding blocking?
>
> add_to_swap_cache itself does not need to be called with GFP_ATOMIC.
>
> There are three places from which it is called:
>
> read_swap_cache_async (typically used when faulting) masks the
> gfp_mask coming in (typically GFP_HIGHUSER_MOVABLE for the pages
> themselves) to call add_to_swap_cache typically with GFP_KERNEL.
>
> shmem_writepage does call it with GFP_ATOMIC: that's because it's
> holding the shmem_inode's spin_lock while it switches the page between
> file cache and swap cache - IIRC holding page lock isn't quite enough
> for that, because of other cases; but I've not thought that through
> in a long time, we could re-examine if it troubles you.

Yes. My point was that. but I am not sure what are other cases. :(
Now, It don't hurt me but not sure in future.

> The questionable one is add_to_swap (when vmscanning), which calls
> it with __GFP_HIGH|__GFP_NOMEMALLOC|__GFP_NOWARN, i.e. GFP_ATOMIC
> plus __GFP_NOMEMALLOC|__GFP_NOWARN.  That one I have wondered
> about from time to time: GFP_NOIO would be the obvious choice,
> that's what swap_writepage will use to allocate bio soon after.
>
> I've been tempted to change it, but afraid to touch that house
> of cards, and afraid of long testing and justification required.
> Would it be safe to drop that __GFP_HIGH?  What's the effect of the
> __GFP_NOMEMALLOC (we've layer on layer of tweak this one way because
> we're in the reclaim path so let it eat more, then tweak it the other
> way because we don't want it to eat up _too_ much).  I just let it stay.

Sigh.
What a complex thing to change one line.
Thanks for kind explanation.

> Hugh
>



-- 
Kinds regards,
Minchan Kim
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