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. At which point, the bulk allocation loop can be simplified because we can only fail bulk allocation for readahead, so something like: if (filled == bp->b_page_count) { XFS_STATS_INC(bp->b_mount, xb_page_found); break; } - if (filled != last) + if (filled == last) { - continue; - - if (flags & XBF_READ_AHEAD) { ASSERT(flags & XBF_READ_AHEAD); xfs_buf_free_pages(bp); return -ENOMEM; } XFS_STATS_INC(bp->b_mount, xb_page_retries); - congestion_wait(BLK_RW_ASYNC, HZ / 50); } return 0; } would do the right thing and still record that we are doing blocking allocations (via the xb_page_retries stat) in this loop. Cheers, Dave. -- Dave Chinner david@xxxxxxxxxxxxx