linux-kernel - Re: [PATCH 06/16] huge tmpfs: shmem_is

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Date:   Fri, 6 Aug 2021 10:57:32 -0700
From:   Yang Shi <shy828301@...il.com>
To:     Hugh Dickins <hughd@...gle.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Shakeel Butt <shakeelb@...gle.com>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Miaohe Lin <linmiaohe@...wei.com>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Michal Hocko <mhocko@...e.com>,
        Rik van Riel <riel@...riel.com>,
        Christoph Hellwig <hch@...radead.org>,
        Matthew Wilcox <willy@...radead.org>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        Alexey Gladkov <legion@...nel.org>,
        Chris Wilson <chris@...is-wilson.co.uk>,
        Matthew Auld <matthew.auld@...el.com>,
        Linux FS-devel Mailing List <linux-fsdevel@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-api@...r.kernel.org, Linux MM <linux-mm@...ck.org>
Subject: Re: [PATCH 06/16] huge tmpfs: shmem_is_huge(vma, inode, index)

On Thu, Aug 5, 2021 at 10:43 PM Hugh Dickins <hughd@...gle.com> wrote:
>
> On Thu, 5 Aug 2021, Yang Shi wrote:
> >
> > By rereading the code, I think you are correct. Both cases do work
> > correctly without leaking. And the !CONFIG_NUMA case may carry the
> > huge page indefinitely.
> >
> > I think it is because khugepaged may collapse memory for another NUMA
> > node in the next loop, so it doesn't make too much sense to carry the
> > huge page, but it may be an optimization for !CONFIG_NUMA case.
>
> Yes, that is its intention.
>
> >
> > However, as I mentioned in earlier email the new pcp implementation
> > could cache THP now, so we might not need keep this convoluted logic
> > anymore. Just free the page if collapse is failed then re-allocate
> > THP. The carried THP might improve the success rate a little bit but I
> > doubt how noticeable it would be, may be not worth for the extra
> > complexity at all.
>
> It would be great if the new pcp implementation is good enough to
> get rid of khugepaged's confusing NUMA=y/NUMA=n differences; and all
> the *hpage stuff too, I hope.  That would be a welcome cleanup.

 The other question is if that optimization is worth it nowadays or
not. I bet not too many users build NUMA=n kernel nowadays even though
the kernel is actually running on a non-NUMA machine. Some small
devices may run NUMA=n kernel, but I don't think they actually use
THP. So such code complexity could be removed from this point of view
too.

>
> > > > Collapse failure is not uncommon and leaking huge pages gets noticed.
>
> After writing that, I realized how I'm almost always testing a NUMA=y
> kernel (though on non-NUMA machines), and seldom try the NUMA=n build.
> So did so to check no leak, indeed; but was surprised, when comparing
> vmstats, that the NUMA=n run had done 5 times as much thp_collapse_alloc
> as the NUMA=y run.  I've merely made a note to look into that one day:
> maybe it was just a one-off oddity, or maybe the incrementing of stats
> is wrong down one path or the other.

Yeah, probably.

>
> Hugh