Re: [PATCH v4 1/6] mm: teach mm by current context info to not do I/O during memory allocation

[Andrew Morton <akpm@linux-foundation.org>]

On Wed, 7 Nov 2012 11:11:24 +0800 Ming Lei <ming.lei@canonical.com> wrote:

> On Wed, Nov 7, 2012 at 7:23 AM, Andrew Morton <akpm@linux-foundation.org> wrote:
> >
> > It's unclear from the description why we're also clearing __GFP_FS in
> > this situation.
> >
> > If we can avoid doing this then there will be a very small gain: there
> > are some situations in which a filesystem can clean pagecache without
> > performing I/O.
> 
> Firstly,  the patch follows the policy in the system suspend/resume situation,
> in which the __GFP_FS is cleared, and basically the problem is very similar
> with that in system PM path.

I suspect that code is wrong.  Or at least, suboptimal.

> Secondly, inside shrink_page_list(), pageout() may be triggered on dirty anon
> page if __GFP_FS is set.

pageout() should be called if GFP_FS is set or if GFP_IO is set and the
IO is against swap.

And that's what we want to happen: we want to enter the fs to try to
turn dirty pagecache into clean pagecache without doing IO.  If we in
fact enter the device drivers when GFP_IO was not set then that's a bug
which we should fix.

> IMO, if performing I/O can be completely avoided when __GFP_FS is set, the
> flag can be kept, otherwise it is better to clear it in the situation.

yup.

> >
> > Also, you can probably put the unlikely() inside memalloc_noio() and
> > avoid repeating it at all the callsites.
> >
> > And it might be neater to do:
> >
> > /*
> >  * Nice comment goes here
> >  */
> > static inline gfp_t memalloc_noio_flags(gfp_t flags)
> > {
> >         if (unlikely(current->flags & PF_MEMALLOC_NOIO))
> >                 flags &= ~GFP_IOFS;
> >         return flags;
> > }
> 
> But without the check in callsites, some local variables will be write
> two times,
> so it is better to not do it.

I don't see why - we just modify the incoming gfp_t at the start of the
function, then use it.

It gets a bit tricky with those struct initialisations.  Things like

	struct foo bar {
		.a = a1,
		.b = b1,
	};

should not be turned into

	struct foo bar {
		.a = a1,
	};
	
	bar.b = b1;

and we don't want to do

	struct foo bar { };

	bar.a = a1;
	bar.b = b1;

either, because these are indeed a double-write.  But we can do

	struct foo bar {
		.flags = (flags = memalloc_noio_flags(flags)),
		.b = b1,
	};

which is a bit arcane but not toooo bad.  Have a think about it...


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