Re: [PATCH v1 05/18] mm: improve folio_likely_mapped_shared() using the mapcount of large folios
Hey David,
Maybe I spotted a bug below.
[...]
static inline bool folio_likely_mapped_shared(struct folio *folio)
{
- return page_mapcount(folio_page(folio, 0)) > 1;
+ int mapcount = folio_mapcount(folio);
+
+ /* Only partially-mappable folios require more care. */
+ if (!folio_test_large(folio) || unlikely(folio_test_hugetlb(folio)))
+ return mapcount > 1;
+
+ /* A single mapping implies "mapped exclusively". */
+ if (mapcount <= 1)
+ return false;
+
+ /* If any page is mapped more than once we treat it "mapped shared". */
+ if (folio_entire_mapcount(folio) || mapcount > folio_nr_pages(folio))
+ return true;
bug: if a PMD-mapped THP is exclusively mapped, the folio_entire_mapcount()
function will return 1 (atomic_read(&folio->_entire_mapcount) + 1).
IIUC, when mapping a PMD entry for the entire THP, folio->_entire_mapcount
increments from -1 to 0.
Thanks,
Lance
+
+ /* Let's guess based on the first subpage. */
+ return atomic_read(&folio->_mapcount) > 0;
}
[...]
Re: [PATCH v1 05/18] mm: improve folio_likely_mapped_shared() using the mapcount of large folios
On 16.04.24 12:40, Lance Yang wrote:
Hey David,
Maybe I spotted a bug below.
Thanks for the review!
[...]
static inline bool folio_likely_mapped_shared(struct folio *folio)
{
- return page_mapcount(folio_page(folio, 0)) > 1;
+ int mapcount = folio_mapcount(folio);
+
+ /* Only partially-mappable folios require more care. */
+ if (!folio_test_large(folio) || unlikely(folio_test_hugetlb(folio)))
+ return mapcount > 1;
+
+ /* A single mapping implies "mapped exclusively". */
+ if (mapcount <= 1)
+ return false;
+
+ /* If any page is mapped more than once we treat it "mapped shared". */
+ if (folio_entire_mapcount(folio) || mapcount > folio_nr_pages(folio))
+ return true;
bug: if a PMD-mapped THP is exclusively mapped, the folio_entire_mapcount()
function will return 1 (atomic_read(&folio->_entire_mapcount) + 1).
If it's exclusively mapped, then folio_mapcount(folio)==1. In which case
the previous statement:
if (mapcount <= 1)
return false;
Catches it.
IOW, once we reach this point we now that folio_mapcount(folio) > 1, and
there must be something else besides the entire mapping ("more than once").
Or did I not address your concern?
--
Cheers,
David / dhildenb
Re: [PATCH v1 05/18] mm: improve folio_likely_mapped_shared() using the mapcount of large folios
On Tue, Apr 16, 2024 at 6:47 PM David Hildenbrand wrote:
>
> On 16.04.24 12:40, Lance Yang wrote:
> > Hey David,
> >
> > Maybe I spotted a bug below.
>
> Thanks for the review!
>
> >
> > [...]
> > static inline bool folio_likely_mapped_shared(struct folio *folio)
> > {
> > - return page_mapcount(folio_page(folio, 0)) > 1;
> > + int mapcount = folio_mapcount(folio);
> > +
> > + /* Only partially-mappable folios require more care. */
> > + if (!folio_test_large(folio) || unlikely(folio_test_hugetlb(folio)))
> > + return mapcount > 1;
> > +
> > + /* A single mapping implies "mapped exclusively". */
> > + if (mapcount <= 1)
> > + return false;
> > +
> > + /* If any page is mapped more than once we treat it "mapped shared".
> > */
> > + if (folio_entire_mapcount(folio) || mapcount > folio_nr_pages(folio))
> > + return true;
> >
> > bug: if a PMD-mapped THP is exclusively mapped, the folio_entire_mapcount()
> > function will return 1 (atomic_read(&folio->_entire_mapcount) + 1).
>
> If it's exclusively mapped, then folio_mapcount(folio)==1. In which case
> the previous statement:
>
> if (mapcount <= 1)
> return false;
>
> Catches it.
You're right!
>
> IOW, once we reach this point we now that folio_mapcount(folio) > 1, and
> there must be something else besides the entire mapping ("more than once").
>
>
> Or did I not address your concern?
Sorry, my mistake :(
Thanks,
Lance
>
> --
> Cheers,
>
> David / dhildenb
>
Re: [PATCH v1 05/18] mm: improve folio_likely_mapped_shared() using the mapcount of large folios
On 16.04.24 12:52, Lance Yang wrote:
On Tue, Apr 16, 2024 at 6:47 PM David Hildenbrand wrote:
On 16.04.24 12:40, Lance Yang wrote:
Hey David,
Maybe I spotted a bug below.
Thanks for the review!
[...]
static inline bool folio_likely_mapped_shared(struct folio *folio)
{
- return page_mapcount(folio_page(folio, 0)) > 1;
+ int mapcount = folio_mapcount(folio);
+
+ /* Only partially-mappable folios require more care. */
+ if (!folio_test_large(folio) || unlikely(folio_test_hugetlb(folio)))
+ return mapcount > 1;
+
+ /* A single mapping implies "mapped exclusively". */
+ if (mapcount <= 1)
+ return false;
+
+ /* If any page is mapped more than once we treat it "mapped shared". */
+ if (folio_entire_mapcount(folio) || mapcount > folio_nr_pages(folio))
+ return true;
bug: if a PMD-mapped THP is exclusively mapped, the folio_entire_mapcount()
function will return 1 (atomic_read(&folio->_entire_mapcount) + 1).
If it's exclusively mapped, then folio_mapcount(folio)==1. In which case
the previous statement:
if (mapcount <= 1)
return false;
Catches it.
You're right!
IOW, once we reach this point we now that folio_mapcount(folio) > 1, and
there must be something else besides the entire mapping ("more than once").
Or did I not address your concern?
Sorry, my mistake :(
No worries, thanks for the review and thinking this through!
--
Cheers,
David / dhildenb
