From: Hugh Dickins <hughd@google.com>
To: David Hildenbrand <david@redhat.com>
Cc: Hugh Dickins <hughd@google.com>,
Yang Shi <yang.shi@linux.alibaba.com>,
kirill.shutemov@linux.intel.com, aarcange@redhat.com,
akpm@linux-foundation.org, linux-mm@kvack.org,
linux-kernel@vger.kernel.org
Subject: Re: [v2 PATCH] mm: shmem: allow split THP when truncating THP partially
Date: Tue, 25 Feb 2020 12:31:07 -0800 (PST) [thread overview]
Message-ID: <alpine.LSU.2.11.2002251216480.7087@eggly.anvils> (raw)
In-Reply-To: <14abd659-1571-8196-202d-d2fcc227a4b0@redhat.com>
On Tue, 25 Feb 2020, David Hildenbrand wrote:
> >
> > I notice that this thread has veered off into QEMU ballooning
> > territory: which may indeed be important, but there's nothing at all
> > that I can contribute on that. I certainly do not want to slow down
> > anything important, but remain convinced that the correct filesystem
> > implementation for punching a hole is to punch a hole.
>
> I am not completely sure I follow all the shmem details (sorry!). But
> trying to "punch a partial hole punch" into a hugetlbfs page will result
> in the very same behavior as with shmem as of now, no?
I believe so.
>
> FALLOC_FL_PUNCH_HOLE: "Within the specified range, partial filesystem
> blocks are zeroed, and whole filesystem blocks are removed from the
> file." ... After a successful call, subsequent reads from this range
> will return zeros."
>
> So, as long as we are talking about partial blocks the documented
> behavior seems to be to only zero the memory.
>
> Does this patch fix "FALLOC_FL_PUNCH_HOLE does not free blocks if called
> in block granularity on shmem" (which would be a valid fix),
Yes. The block size of tmpfs is (talking x86_64 for simplicity) 4KiB;
but when mounted huge, it transparently takes advantage of 2MiB extents
when it can. Rather like a disk-based filesystem always presenting a
4KiB block interface, but stored on disk in multisector extents.
Whereas hugetlbfs is a different filesystem, which is and always has
been limited to supporting only certain larger block sizes.
> or does it
> try to implement something that is not documented? (removing partial
> blocks when called in sub-block granularity)
No.
>
> I assume the latter, in which case I would interpret "punching a hole is
> to punch a hole" as "punching sub-blocks will not free blocks".
>
> (if somebody could enlighten me which important piece I am missing or
> messing up, that would be great :) )
>
> --
> Thanks,
>
> David / dhildenb
next prev parent reply other threads:[~2020-02-25 20:31 UTC|newest]
Thread overview: 30+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-12-04 0:42 [v2 PATCH] mm: shmem: allow split THP when truncating THP partially Yang Shi
2019-12-05 0:15 ` Hugh Dickins
2019-12-05 0:50 ` Yang Shi
2020-01-14 19:28 ` Yang Shi
2020-02-04 23:27 ` Yang Shi
2020-02-14 0:38 ` Yang Shi
2020-02-14 15:40 ` Kirill A. Shutemov
2020-02-14 17:17 ` Yang Shi
2020-02-25 3:46 ` Hugh Dickins
2020-02-25 18:02 ` David Hildenbrand
2020-02-25 20:31 ` Hugh Dickins [this message]
2020-02-26 17:43 ` Yang Shi
2020-02-27 1:16 ` Matthew Wilcox
2020-02-27 1:47 ` Hugh Dickins
2020-02-27 1:37 ` Hugh Dickins
2020-02-20 18:16 ` Alexander Duyck
2020-02-21 9:07 ` Michael S. Tsirkin
2020-02-21 9:36 ` David Hildenbrand
2020-02-22 0:39 ` Alexander Duyck
2020-02-24 10:22 ` David Hildenbrand
2020-02-25 0:13 ` Alexander Duyck
2020-02-25 8:09 ` David Hildenbrand
2020-02-25 16:42 ` Alexander Duyck
2020-02-21 18:24 ` Yang Shi
2020-02-22 0:24 ` Alexander Duyck
2020-02-26 17:31 ` Yang Shi
2020-02-26 17:45 ` David Hildenbrand
2020-02-26 18:00 ` Yang Shi
2020-02-27 0:56 ` Hugh Dickins
2020-02-27 1:14 ` Yang Shi
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