linux-kernel - Re: [PATCH v2 2/2] smaps: set THPeligible if file mapping supports large folios

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Message-ID: <ZN/t4sgCe5lsFUpR@casper.infradead.org>
Date:   Fri, 18 Aug 2023 23:17:06 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     Zach O'Keefe <zokeefe@...gle.com>
Cc:     linux-mm@...ck.org, Yang Shi <shy828301@...il.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 2/2] smaps: set THPeligible if file mapping supports
 large folios

On Fri, Aug 18, 2023 at 03:14:02PM -0700, Zach O'Keefe wrote:
> Sorry -- noticed only too late that there are still many
> false-negatives for THPeligible, since by this point in the function
> we've already applied sysfs and prctl restrictions, which file-fault
> ignores. VM_HUGEPAGE also needs to be checked for the file-fault case.

I'm not entirely convinced that unifying all of these things leads
to code that's simpler to understand.

> On Fri, Aug 18, 2023 at 2:15 PM Zach O'Keefe <zokeefe@...gle.com> wrote:
> >
> > File-backed memory can be backed by THPs either through collapse, when
> > CONFIG_READ_ONLY_THP_FOR_FS is enabled, or through fault, when the
> > filesystem supports large folio mappings.
> >
> > Currently, smaps only knows about the former, so teach it about the
> > latter.
> >
> > Signed-off-by: Zach O'Keefe <zokeefe@...gle.com>
> > Cc: Matthew Wilcox <willy@...radead.org>
> > ---
> >  mm/huge_memory.c | 11 ++++++++++-
> >  1 file changed, 10 insertions(+), 1 deletion(-)
> >
> > diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> > index cd379b2c077b..d8d6e83820f3 100644
> > --- a/mm/huge_memory.c
> > +++ b/mm/huge_memory.c
> > @@ -136,7 +136,16 @@ bool hugepage_vma_check(struct vm_area_struct *vma, unsigned long vm_flags,
> >                          */
> >                         !!vma->vm_ops->huge_fault :
> >                         /* Only regular file is valid in collapse path */
> > -                       file_thp_enabled(vma);
> > +                       file_thp_enabled(vma) ||
> > +                        /*
> > +                         * THPeligible bit of smaps should surface the
> > +                         * possibility of THP through fault if the filesystem
> > +                         * supports it.  We don't check this in fault path,
> > +                         * because we want to fallback to the actual ->fault()
> > +                         * handler to make the decision.
> > +                         */
> > +                        (smaps && vma->vm_file &&
> > +                        mapping_large_folio_support(vma->vm_file->f_mapping));
> >
> >         if (vma_is_temporary_stack(vma))
> >                 return false;
> > --
> > 2.42.0.rc1.204.g551eb34607-goog
> >