linux-kernel - Re: [PATCH 2/3] Retry page fault when blocking on disk transfer.

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Date:	Fri, 08 Oct 2010 09:24:32 -0400
From:	Rik van Riel <riel@...hat.com>
To:	Michel Lespinasse <walken@...gle.com>
CC:	Linus Torvalds <torvalds@...ux-foundation.org>, linux-mm@...ck.org,
	Ying Han <yinghan@...gle.com>, linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Nick Piggin <npiggin@...nel.dk>,
	Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH 2/3] Retry page fault when blocking on disk transfer.

On 10/08/2010 12:39 AM, Michel Lespinasse wrote:
> On Tue, Oct 05, 2010 at 03:44:22PM -0700, Michel Lespinasse wrote:
>> On Tue, Oct 5, 2010 at 10:38 AM, Rik van Riel<riel@...hat.com>  wrote:
>>> Looks like it should be relatively easy to do something
>>> similar in do_swap_page also.
>>
>> Good idea. We don't make use of swap too much, which is probably why
>> we didn't have that in our kernel, but it seems like a good idea just
>> for uniformity. I'll add this in a follow-on patch.
>
> So here's the patch. Sorry for the delay - it did not take long to write,
> but I couldn't test it before today.
>
> Please have a look - I'd like to add this to the series I sent earlier.
>
> ----------------------------------- 8<  ---------------------------------
>
> Retry page fault when blocking on swap in
>
> This change is the cousin of 'Retry page fault when blocking
> on disk transfer'. The idea here is to reduce mmap_sem hold times
> that are caused by disk transfers when swapping in pages. We drop
> mmap_sem while waiting for the page lock, and return the VM_FAULT_RETRY
> flag. do_page_fault will then re-acquire mmap_sem and retry the
> page fault. It is expected that upon retry the page will now be cached,
> and thus the retry will complete with a low mmap_sem hold time.
>
> Signed-off-by: Michel Lespinasse<walken@...gle.com>
>
> diff --git a/mm/memory.c b/mm/memory.c
> index b068c68..0ec70b4 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -2613,6 +2613,21 @@ int vmtruncate_range(struct inode *inode, loff_t offset, loff_t end)
>   	return 0;
>   }
>
> +static inline int lock_page_or_retry(struct page *page, struct mm_struct *mm,
> +				     unsigned int flags)
> +{
> +	if (trylock_page(page))
> +		return 1;
> +	if (!(flags&  FAULT_FLAG_ALLOW_RETRY)) {
> +		__lock_page(page);
> +		return 1;
> +	}
> +
> +	up_read(&mm->mmap_sem);
> +	wait_on_page_locked(page);
> +	return 0;
> +}

Wait a moment.  Your other patch 2/3 also has a
lock_page_or_retry function.  That one is in
filemap.c and takes slightly different arguments,
to do essentially the same thing...

+/*
+ * Lock the page, unless this would block and the caller indicated that it
+ * can handle a retry.
+ */
+static int lock_page_or_retry(struct page *page,
+			      struct vm_area_struct *vma, struct vm_fault *vmf)
+{

Is there a way the two functions can be merged
into one?

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