Re: [PATCH 06/16] huge tmpfs: shmem_is_huge(vma, inode, index)

[Yang Shi]

On Thu, Aug 5, 2021 at 10:21 PM Hugh Dickins <hughd@xxxxxxxxxx> wrote:
>
> On Wed, 4 Aug 2021, Yang Shi wrote:
> > On Wed, Aug 4, 2021 at 1:28 AM Hugh Dickins <hughd@xxxxxxxxxx> 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 outdated "bring it back from swap" comment - swap got done above.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