Re: [PATCH v4 03/11] filemap: allocate mapping_min_order folios in the page cache

Pankaj Raghav (Samsung)
2024-04-28T20:59:24Z

On Fri, Apr 26, 2024 at 08:12:43AM -0700, Darrick J. Wong wrote:
> On Thu, Apr 25, 2024 at 01:37:38PM +0200, Pankaj Raghav (Samsung) wrote:
> > From: Luis Chamberlain <mcgrof@kernel.org>
> >
> > filemap_create_folio() and do_read_cache_folio() were always allocating
> > folio of order 0. __filemap_get_folio was trying to allocate higher
> > order folios when fgp_flags had higher order hint set but it will default
> > to order 0 folio if higher order memory allocation fails.
> >
> > Supporting mapping_min_order implies that we guarantee each folio in the
> > page cache has at least an order of mapping_min_order. When adding new
> > folios to the page cache we must also ensure the index used is aligned to
> > the mapping_min_order as the page cache requires the index to be aligned
> > to the order of the folio.
>
> If we cannot find a folio of at least min_order size, what error is sent
> back?
>
> If the answer is "the same error that you get if we cannot allocate a
> base page today (aka ENOMEM)", then I think I understand this enough to
> say
>
> Reviewed-by: Darrick J. Wong <djwong@kernel.org>

Yes. We will get a ENOMEM if we cannot allocate min_order size folio. :)
Thanks!
>
> --D
>
> > Signed-off-by: Luis Chamberlain <mcgrof@kernel.org>
> > Co-developed-by: Pankaj Raghav <p.raghav@samsung.com>
> > Signed-off-by: Pankaj Raghav <p.raghav@samsung.com>
> > ---
> > mm/filemap.c | 24 +++++++++++++++++-------
> > 1 file changed, 17 insertions(+), 7 deletions(-)
> >
> > diff --git a/mm/filemap.c b/mm/filemap.c
> > index 30de18c4fd28..f0c0cfbbd134 100644
> > --- a/mm/filemap.c
> > +++ b/mm/filemap.c
> > @@ -858,6 +858,8 @@ noinline int __filemap_add_folio(struct address_space *mapping,
> >
> > VM_BUG_ON_FOLIO(!folio_test_locked(folio), folio);
> > VM_BUG_ON_FOLIO(folio_test_swapbacked(folio), folio);
> > + VM_BUG_ON_FOLIO(folio_order(folio) < mapping_min_folio_order(mapping),
> > + folio);
> > mapping_set_update(&xas, mapping);
> >
> > if (!huge) {
> > @@ -1895,8 +1897,10 @@ struct folio *__filemap_get_folio(struct address_space *mapping, pgoff_t index,
> > folio_wait_stable(folio);
> > no_page:
> > if (!folio && (fgp_flags & FGP_CREAT)) {
> > - unsigned order = FGF_GET_ORDER(fgp_flags);
> > + unsigned int min_order = mapping_min_folio_order(mapping);
> > + unsigned int order = max(min_order, FGF_GET_ORDER(fgp_flags));
> > int err;
> > + index = mapping_align_start_index(mapping, index);
> >
> > if ((fgp_flags & FGP_WRITE) && mapping_can_writeback(mapping))
> > gfp |= __GFP_WRITE;
> > @@ -1936,7 +1940,7 @@ struct folio *__filemap_get_folio(struct address_space *mapping, pgoff_t index,
> > break;
> > folio_put(folio);
> > folio = NULL;
> > - } while (order-- > 0);
> > + } while (order-- > min_order);
> >
> > if (err == -EEXIST)
> > goto repeat;
> > @@ -2425,13 +2429,16 @@ static int filemap_update_page(struct kiocb *iocb,
> > }
> >
> > static int filemap_create_folio(struct file *file,
> > - struct address_space *mapping, pgoff_t index,
> > + struct address_space *mapping, loff_t pos,
> > struct folio_batch *fbatch)
> > {
> > struct folio *folio;
> > int error;
> > + unsigned int min_order = mapping_min_folio_order(mapping);
> > + pgoff_t index;
> >
> > - folio = filemap_alloc_folio(mapping_gfp_mask(mapping), 0);
> > + folio = filemap_alloc_folio(mapping_gfp_mask(mapping),
> > + min_order);
> > if (!folio)
> > return -ENOMEM;
> >
> > @@ -2449,6 +2456,8 @@ static int filemap_create_folio(struct file *file,
> > * well to keep locking rules simple.
> > */
> > filemap_invalidate_lock_shared(mapping);
> > + /* index in PAGE units but aligned to min_order number of pages. */
> > + index = (pos >> (PAGE_SHIFT + min_order)) << min_order;
> > error = filemap_add_folio(mapping, folio, index,
> > mapping_gfp_constraint(mapping, GFP_KERNEL));
> > if (error == -EEXIST)
> > @@ -2509,8 +2518,7 @@ static int filemap_get_pages(struct kiocb *iocb, size_t count,
> > if (!folio_batch_count(fbatch)) {
> > if (iocb->ki_flags & (IOCB_NOWAIT | IOCB_WAITQ))
> > return -EAGAIN;
> > - err = filemap_create_folio(filp, mapping,
> > - iocb->ki_pos >> PAGE_SHIFT, fbatch);
> > + err = filemap_create_folio(filp, mapping, iocb->ki_pos, fbatch);
> > if (err == AOP_TRUNCATED_PAGE)
> > goto retry;
> > return err;
> > @@ -3708,9 +3716,11 @@ static struct folio *do_read_cache_folio(struct address_space *mapping,
> > repeat:
> > folio = filemap_get_folio(mapping, index);
> > if (IS_ERR(folio)) {
> > - folio = filemap_alloc_folio(gfp, 0);
> > + folio = filemap_alloc_folio(gfp,
> > + mapping_min_folio_order(mapping));
> > if (!folio)
> > return ERR_PTR(-ENOMEM);
> > + index = mapping_align_start_index(mapping, index);
> > err = filemap_add_folio(mapping, folio, index, gfp);
> > if (unlikely(err)) {
> > folio_put(folio);
> > --
> > 2.34.1
> >
> >