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From: Yang Shi <shy828301@gmail.com>
To: Hugh Dickins <hughd@google.com>
Cc: Andrew Morton <akpm@linux-foundation.org>,
	Shakeel Butt <shakeelb@google.com>,
	"Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
	Miaohe Lin <linmiaohe@huawei.com>,
	Mike Kravetz <mike.kravetz@oracle.com>,
	Michal Hocko <mhocko@suse.com>, Rik van Riel <riel@surriel.com>,
	Christoph Hellwig <hch@infradead.org>,
	Matthew Wilcox <willy@infradead.org>,
	"Eric W. Biederman" <ebiederm@xmission.com>,
	Alexey Gladkov <legion@kernel.org>,
	Chris Wilson <chris@chris-wilson.co.uk>,
	Matthew Auld <matthew.auld@intel.com>,
	Linux FS-devel Mailing List <linux-fsdevel@vger.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	linux-api@vger.kernel.org, Linux MM <linux-mm@kvack.org>
Subject: Re: [PATCH 06/16] huge tmpfs: shmem_is_huge(vma, inode, index)
Date: Fri, 6 Aug 2021 10:41:49 -0700	[thread overview]
Message-ID: <CAHbLzkre9PS_sSo6nQGu5rpsNW=FQ2jvXCMpzmB0OBMT46m87g@mail.gmail.com> (raw)
In-Reply-To: <5add2467-3b23-f8b8-e07b-82d8a573ecb7@google.com>

On Thu, Aug 5, 2021 at 10:21 PM Hugh Dickins <hughd@google.com> wrote:
>
> On Wed, 4 Aug 2021, Yang Shi wrote:
> > On Wed, Aug 4, 2021 at 1:28 AM Hugh Dickins <hughd@google.com> wrote:
> > >
> > > Thanks, but despite us agreeing that the race is too unlikely to be worth
> > > optimizing against, it does still nag at me ever since you questioned it:
> > > silly, but I can't quite be convinced by my own dismissals.
> > >
> > > I do still want to get rid of SGP_HUGE and SGP_NOHUGE, clearing up those
> > > huge allocation decisions remains the intention; but now think to add
> > > SGP_NOALLOC for collapse_file() in place of SGP_NOHUGE or SGP_CACHE -
> > > to rule out that possibility of mischarge after racing hole-punch,
> > > no matter whether it's huge or small.  If any such race occurs,
> > > collapse_file() should just give up.
> > >
> > > This being the "Stupid me" SGP_READ idea, except that of course would
> > > not work: because half the point of that block in collapse_file() is
> > > to initialize the !Uptodate pages, whereas SGP_READ avoids doing so.
> > >
> > > There is, of course, the danger that in fixing this unlikely mischarge,
> > > I've got the code wrong and am introducing a bug: here's what a 17/16
> > > would look like, though it will be better inserted early.  I got sick
> > > of all the "if (page "s, and was glad of the opportunity to fix that
> > > outdated "bring it back from swap" comment - swap got done above.
> > >
> > > What do you think? Should I add this in or leave it out?
> >
> > Thanks for keeping investigating this. The patch looks good to me. I
> > think we could go this way. Just a nit below.
>
> Thanks, I'll add it into the series, a patch before SGP_NOHUGE goes away;
> but I'm not intending to respin the series until there's more feedback
> from others - fcntl versus fadvise is the main issue so far.

Thanks, yeah, no hurry to repost.

>
> > > --- a/include/linux/shmem_fs.h
> > > +++ b/include/linux/shmem_fs.h
> > > @@ -108,6 +108,7 @@ extern unsigned long shmem_partial_swap_usage(struct address_space *mapping,
> > >  /* Flag allocation requirements to shmem_getpage */
> > >  enum sgp_type {
> > >         SGP_READ,       /* don't exceed i_size, don't allocate page */
> > > +       SGP_NOALLOC,    /* like SGP_READ, but do use fallocated page */
> >
> > The comment looks misleading, it seems SGP_NOALLOC does clear the
> > Uptodate flag but SGP_READ doesn't. Or it is fine not to distinguish
> > this difference?
>
> I think you meant to say, SGP_NOALLOC does *set* the Uptodate flag but
> SGP_READ doesn't.  And a more significant difference, as coded to suit
> collapse_file(), is that SGP_NOALLOC returns failure on hole, whereas
> SGP_READ returns success: I should have mentioned that.

Yes, I mean "set". Sorry for the confusion.

>
> When I wrote "like SGP_READ" there, I just meant "like what's said in
> the line above": would "ditto" be okay with you, and I say
>         SGP_NOALLOC,    /* ditto, but fail on hole, or use fallocated page */
>
> I don't really want to get into the "Uptodate" business there.
> And I'm afraid someone is going to ask me to write multi-line comments
> on each of those SGP_flags, and I'm going to plead "read the source"!

OK, I'm fine as is.

>
> Oh, now I see why you said SGP_NOALLOC does clear the Uptodate flag:
> "goto clear", haha: when we clear the page we set the Uptodate flag.
>
> And I may have another patch to slot in: I was half expecting you to
> question why SGP_READ behaves as it does, so in preparing its defence
> I checked, and found it was not doing quite what I remembered: changes
> were made a long time ago, which have left it slightly suboptimal.
> But that really has nothing to do with the rest of this series,
> and I don't need to run it past you before reposting.
>
> I hope that some of the features in this series can be useful to you.

Thanks, I will see.

>
> Thanks,
> Hugh

  reply	other threads:[~2021-08-06 17:42 UTC|newest]

Thread overview: 45+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-07-30  7:22 [PATCH 00/16] tmpfs: HUGEPAGE and MEM_LOCK fcntls and memfds Hugh Dickins
2021-07-30  7:25 ` [PATCH 01/16] huge tmpfs: fix fallocate(vanilla) advance over huge pages Hugh Dickins
2021-07-30 21:36   ` Yang Shi
2021-08-01  3:38     ` Hugh Dickins
2021-08-02 20:36       ` Yang Shi
2021-07-30  7:28 ` [PATCH 02/16] huge tmpfs: fix split_huge_page() after FALLOC_FL_KEEP_SIZE Hugh Dickins
2021-07-30 23:48   ` Yang Shi
2021-07-30  7:30 ` [PATCH 03/16] huge tmpfs: remove shrinklist addition from shmem_setattr() Hugh Dickins
2021-07-30 21:50   ` Yang Shi
2021-07-30  7:36 ` [PATCH 04/16] huge tmpfs: revert shmem's use of transhuge_vma_enabled() Hugh Dickins
2021-07-30 21:56   ` Yang Shi
2021-08-01  4:01     ` Hugh Dickins
2021-08-02 20:39       ` Yang Shi
2021-07-30  7:39 ` [PATCH 05/16] huge tmpfs: move shmem_huge_enabled() upwards Hugh Dickins
2021-07-30 21:57   ` Yang Shi
2021-07-30  7:42 ` [PATCH 06/16] huge tmpfs: shmem_is_huge(vma, inode, index) Hugh Dickins
2021-07-30 23:34   ` Yang Shi
2021-08-01  5:22     ` Hugh Dickins
2021-08-01  5:37       ` Hugh Dickins
2021-08-02 21:14       ` Yang Shi
2021-08-04  8:28         ` Hugh Dickins
2021-08-04 19:01           ` Yang Shi
2021-08-06  5:21             ` Hugh Dickins
2021-08-06 17:41               ` Yang Shi [this message]
2021-08-05 23:04         ` Yang Shi
2021-08-06  5:43           ` Hugh Dickins
2021-08-06 17:57             ` Yang Shi
2021-08-12 18:19               ` Yang Shi
2021-07-30  7:45 ` [PATCH 07/16] memfd: memfd_create(name, MFD_HUGEPAGE) for shmem huge pages Hugh Dickins
2021-08-04 14:03   ` Kirill A. Shutemov
2021-08-06  3:33     ` Hugh Dickins
2021-07-30  7:48 ` [PATCH 08/16] huge tmpfs: fcntl(fd, F_HUGEPAGE) and fcntl(fd, F_NOHUGEPAGE) Hugh Dickins
2021-08-04 14:08   ` Kirill A. Shutemov
2021-08-06  4:34     ` Hugh Dickins
2021-07-30  7:51 ` [PATCH 09/16] huge tmpfs: decide stat.st_blksize by shmem_is_huge() Hugh Dickins
2021-07-30 23:40   ` Yang Shi
2021-07-30  7:55 ` [PATCH 10/16] tmpfs: fcntl(fd, F_MEM_LOCK) to memlock a tmpfs file Hugh Dickins
2021-08-03  1:38   ` Matthew Wilcox
2021-08-04  9:15     ` Hugh Dickins
2021-07-30  7:57 ` [PATCH 11/16] tmpfs: fcntl(fd, F_MEM_LOCKED) to test if memlocked Hugh Dickins
2021-07-30  8:00 ` [PATCH 12/16] tmpfs: refuse memlock when fallocated beyond i_size Hugh Dickins
2021-07-30  8:03 ` [PATCH 13/16] mm: bool user_shm_lock(loff_t size, struct ucounts *) Hugh Dickins
2021-07-30  8:06 ` [PATCH 14/16] mm: user_shm_lock(,,getuc) and user_shm_unlock(,,putuc) Hugh Dickins
2021-07-30  8:09 ` [PATCH 15/16] tmpfs: permit changing size of memlocked file Hugh Dickins
2021-07-30  8:13 ` [PATCH 16/16] memfd: memfd_create(name, MFD_MEM_LOCK) for memlocked shmem Hugh Dickins

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