On Wed, Nov 7, 2012 at 11:48 AM, Andrew Morton
[off-list ref] wrote:

quoted

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.

quoted

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.

quoted

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.

quoted

quoted

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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