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

[Hugh Dickins <hughd@google.com>]

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 outdated "bring it back from swap" comment - swap got done above.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.

> > --- 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.

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"!

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,
Hugh