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

[Ming Lei <ming.lei@canonical.com>]

On Wed, Nov 7, 2012 at 11:48 AM, Andrew Morton
<akpm@linux-foundation.org> wrote:
>>
>> 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.

OK, I got it, and I'll not clear GFP_FS in -v5.

>
>> 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...

Got it, looks memalloc_noio_flags() neater, and I will take it in v5.

Thanks,
--
Ming Lei

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