linux-kernel - Re: [PATCH 5/7] mm/gup: Cleanup next

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Message-ID: <ZJCp9aBS8INMkehh@x1n>
Date:   Mon, 19 Jun 2023 15:18:13 -0400
From:   Peter Xu <peterx@...hat.com>
To:     Lorenzo Stoakes <lstoakes@...il.com>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        Matthew Wilcox <willy@...radead.org>,
        Andrea Arcangeli <aarcange@...hat.com>,
        John Hubbard <jhubbard@...dia.com>,
        Mike Rapoport <rppt@...nel.org>,
        David Hildenbrand <david@...hat.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        "Kirill A . Shutemov" <kirill@...temov.name>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        James Houghton <jthoughton@...gle.com>,
        Hugh Dickins <hughd@...gle.com>
Subject: Re: [PATCH 5/7] mm/gup: Cleanup next_page handling

On Sat, Jun 17, 2023 at 09:00:34PM +0100, Lorenzo Stoakes wrote:
> On Sat, Jun 17, 2023 at 08:48:38PM +0100, Lorenzo Stoakes wrote:
> > On Tue, Jun 13, 2023 at 05:53:44PM -0400, Peter Xu wrote:
> > > The only path that doesn't use generic "**pages" handling is the gate vma.
> > > Make it use the same path, meanwhile tune the next_page label upper to
> > > cover "**pages" handling.  This prepares for THP handling for "**pages".
> > >
> > > Signed-off-by: Peter Xu <peterx@...hat.com>
> > > ---
> > >  mm/gup.c | 7 +++----
> > >  1 file changed, 3 insertions(+), 4 deletions(-)
> > >
> > > diff --git a/mm/gup.c b/mm/gup.c
> > > index 8d59ae4554e7..a2d1b3c4b104 100644
> > > --- a/mm/gup.c
> > > +++ b/mm/gup.c
> > > @@ -1135,7 +1135,7 @@ static long __get_user_pages(struct mm_struct *mm,
> > >  			if (!vma && in_gate_area(mm, start)) {
> > >  				ret = get_gate_page(mm, start & PAGE_MASK,
> > >  						gup_flags, &vma,
> > > -						pages ? &pages[i] : NULL);
> > > +						pages ? &page : NULL);
> >
> > Good spot... ugh that we handled this differently.
> >
> > >  				if (ret)
> > >  					goto out;
> > >  				ctx.page_mask = 0;
> >
> > We can drop this line now right? As the new next_page block will duplicate
> > this.
> 
> OK I can see why you left this in given the last patch in the series :)
> Please disregard.

Yes the other "page_mask=0" will be removed in the next (not last) patch.

> 
> >
> > > @@ -1205,19 +1205,18 @@ static long __get_user_pages(struct mm_struct *mm,
> > >  				ret = PTR_ERR(page);
> > >  				goto out;
> > >  			}
> > > -
> > > -			goto next_page;
> >
> > This is neat, we've already checked if pages != NULL so the if (pages)
> > block at the new next_page label will not be run.

Yes.

> >
> > >  		} else if (IS_ERR(page)) {
> > >  			ret = PTR_ERR(page);
> > >  			goto out;
> > >  		}
> > > +next_page:
> > >  		if (pages) {
> > >  			pages[i] = page;
> > >  			flush_anon_page(vma, page, start);
> > >  			flush_dcache_page(page);
> >
> > I guess there's no harm that we now flush here, though it seems to me to be
> > superfluous, it's not a big deal I don't think.

I'd say GUP on gate vma page should be so rare so yeah I think it shouldn't
be a big deal.  Even iiuc vsyscall=xonly should be the default, so gup may
have already failed on a gate vma page even trying to read-only..

> >
> > >  			ctx.page_mask = 0;
> > >  		}
> > > -next_page:
> > > +
> > >  		page_increm = 1 + (~(start >> PAGE_SHIFT) & ctx.page_mask);
> > >  		if (page_increm > nr_pages)
> > >  			page_increm = nr_pages;
> > > --
> > > 2.40.1
> > >
> >
> > Other than that, LGTM,
> >
> > Reviewed-by: Lorenzo Stoakes <lstoakes@...il.com>

Thanks for looking!

-- 
Peter Xu