On Fri, Feb 04, 2022 at 01:13:29PM -0800, John Hubbard wrote: > On 2/4/22 11:57, Matthew Wilcox (Oracle) wrote: > > We should always increase the refcount before doing anything else to > > the page so that other page users see the elevated refcount first. > > Absolutely agree in principle. Is there anything else to say, though, > such as why this matters here? Or is the change just being done for > "best practices"? (Which is still a very solid reason, of course.) I'm not sure if any software examines both refcount and pincount to determine whether it knows what all of the refcounts to a page mean, but a human might. If someone else calls dump_page() after we update pincount and before we update refcount, we might make a bad decision while debugging. When the updates are the other way around, a spurious extra refcount is to be expected and should not confuse anyone more than they already are. > > Signed-off-by: Matthew Wilcox (Oracle) <willy@xxxxxxxxxxxxx> > > --- > > mm/gup.c | 12 ++++++------ > > 1 file changed, 6 insertions(+), 6 deletions(-) > > > > diff --git a/mm/gup.c b/mm/gup.c > > index a9d4d724aef7..08020987dfc0 100644 > > --- a/mm/gup.c > > +++ b/mm/gup.c > > @@ -220,18 +220,18 @@ bool __must_check try_grab_page(struct page *page, unsigned int flags) > > if (WARN_ON_ONCE(page_ref_count(page) <= 0)) > > return false; > > - if (hpage_pincount_available(page)) > > - hpage_pincount_add(page, 1); > > - else > > - refs = GUP_PIN_COUNTING_BIAS; > > - > > /* > > * Similar to try_grab_compound_head(): even if using the > > * hpage_pincount_add/_sub() routines, be sure to > > * *also* increment the normal page refcount field at least > > * once, so that the page really is pinned. > > */ > > - page_ref_add(page, refs); > > A fine point: this hunk removes the last use of "refs", which means that > this patch will lead to an unused variable warning. So I think it would > be best to remove the "int refs = 1;" line in this patch, rather than > waiting until patch #10. Argh! I noticed that I needed to do that, and then forgot to do it. Thanks! > With that change, please feel free to add: > > Reviewed-by: John Hubbard <jhubbard@xxxxxxxxxx> Added, and git tree updated.
