Re: [PATCH] mm: hugetlbfs: fix hugetlbfs optimization

[Pravin Shelar <pshelar@nicira.com>]

On Tue, Nov 5, 2013 at 2:10 PM, Andrea Arcangeli <aarcange@redhat.com> wrote:
> Hi,
>
> this patch is an alternative implementation of the hugetlbfs directio
> optimization discussed earlier. We've been looking into this with
> Khalid last week and an earlier version of this patch (fully
> equivalent as far as CPU overhead is concerned) was benchmarked by
> Khalid and it didn't degrade performance compared to the PageHuge
> check in current upstream code, so we should be good.
>
> The patch applies cleanly only after reverting
> 7cb2ef56e6a8b7b368b2e883a0a47d02fed66911, it's much easier to review
> it in this form as it avoids all the alignment changes. I'll resend to
> Andrew against current upstream by squashing it with the revert after
> reviews.
>
> I wished to remove the _mapcount tailpage refcounting for slab and
> hugetlbfs tails too, but if the last put_page of a slab tail happens
> after the slab page isn't a slab page anymore (but still compound as
> it wasn't freed yet because of the tail pin), a VM_BUG_ON would
> trigger during the last (unpinning) put_page(slab_tail) with the
> mapcount underflow:
>
>                         VM_BUG_ON(page_mapcount(page) <= 0);
>
> Not even sure if any driver is doing anything like that, but the
> current code would allow it, Pravin should know more about when
> exactly in which conditions the last put_page is done on slab tail
> pages.
>
Yes, This can happen when slab object is directly passed for IO and it
is done in few filesystems (ocfs, xfs) when I checked last time.

> It shall be possible to remove the _mapcount refcounting anyway, as it
> is only read by split_huge_page and so it doesn't actually matter if
> it underflows, but I prefer to keep the VM_BUG_ON. In fact I added one
> more VM_BUG_ON(!PageHead()) even in this patch.

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