On Wed, 15 Sep 2021, Mel Gorman wrote: > On Tue, Sep 14, 2021 at 12:35:59PM +1000, NeilBrown wrote: > > On Tue, 14 Sep 2021, Dave Chinner wrote: > > > On Tue, Sep 14, 2021 at 10:13:04AM +1000, NeilBrown wrote: > > > > Documentation commment in gfp.h discourages indefinite retry loops on > > > > ENOMEM and says of __GFP_NOFAIL that it > > > > > > > > is definitely preferable to use the flag rather than opencode > > > > endless loop around allocator. > > > > > > > > congestion_wait() is indistinguishable from > > > > schedule_timeout_uninterruptible() in practice and it is not a good way > > > > to wait for memory to become available. > > > > > > > > So instead of waiting, allocate a single page using __GFP_NOFAIL, then > > > > loop around and try to get any more pages that might be needed with a > > > > bulk allocation. This single-page allocation will wait in the most > > > > appropriate way. > > > > > > > > Signed-off-by: NeilBrown <neilb@xxxxxxx> > > > > --- > > > > fs/xfs/xfs_buf.c | 6 +++--- > > > > 1 file changed, 3 insertions(+), 3 deletions(-) > > > > > > > > diff --git a/fs/xfs/xfs_buf.c b/fs/xfs/xfs_buf.c > > > > index 5fa6cd947dd4..1ae3768f6504 100644 > > > > --- a/fs/xfs/xfs_buf.c > > > > +++ b/fs/xfs/xfs_buf.c > > > > @@ -372,8 +372,8 @@ xfs_buf_alloc_pages( > > > > > > > > /* > > > > * Bulk filling of pages can take multiple calls. Not filling the entire > > > > - * array is not an allocation failure, so don't back off if we get at > > > > - * least one extra page. > > > > + * array is not an allocation failure, so don't fail or fall back on > > > > + * __GFP_NOFAIL if we get at least one extra page. > > > > */ > > > > for (;;) { > > > > long last = filled; > > > > @@ -394,7 +394,7 @@ xfs_buf_alloc_pages( > > > > } > > > > > > > > XFS_STATS_INC(bp->b_mount, xb_page_retries); > > > > - congestion_wait(BLK_RW_ASYNC, HZ / 50); > > > > + bp->b_pages[filled++] = alloc_page(gfp_mask | __GFP_NOFAIL); > > > > > > This smells wrong - the whole point of using the bulk page allocator > > > in this loop is to avoid the costly individual calls to > > > alloc_page(). > > > > > > What we are implementing here fail-fast semantics for readahead and > > > fail-never for everything else. If the bulk allocator fails to get > > > a page from the fast path free lists, it already falls back to > > > __alloc_pages(gfp, 0, ...) to allocate a single page. So AFAICT > > > there's no need to add another call to alloc_page() because we can > > > just do this instead: > > > > > > if (flags & XBF_READ_AHEAD) > > > gfp_mask |= __GFP_NORETRY; > > > else > > > - gfp_mask |= GFP_NOFS; > > > + gfp_mask |= GFP_NOFS | __GFP_NOFAIL; > > > > > > Which should make the __alloc_pages() call in > > > alloc_pages_bulk_array() do a __GFP_NOFAIL allocation and hence > > > provide the necessary never-fail guarantee that is needed here. > > > > That is a nice simplification. > > Mel Gorman told me > > https://lore.kernel.org/linux-nfs/20210907153116.GJ3828@xxxxxxxx/ > > that alloc_pages_bulk ignores GFP_NOFAIL. I added that to the > > documentation comment in an earlier patch. > > > > I had a look at the code and cannot see how it would fail to allocate at > > least one page. Maybe Mel can help.... > > > > If there are already at least one page an the array and the first attempt > at bulk allocation fails, it'll simply return. It's an odd corner case > that may never apply but it's possible. That said, I'm of the opinion that > __GFP_NOFAIL should not be expanded and instead congestion_wait should be > deleted and replaced with something triggered by reclaim making progress. Ahh.... that was (I think) fixed by https://patchwork.kernel.org/project/linux-mm/patch/163027609524.7591.4987241695872857175@xxxxxxxxxxxxxxxxxxxxx/ (which I cannot find on lore.kernel.org - strange) which you acked - and which I meant to include in this series but somehow missed. NeilBrown