From: Hugh Dickins <hughd@google.com> To: Nick Piggin <npiggin@kernel.dk> Cc: Andrew Morton <akpm@linux-foundation.org>, Peter Zijlstra <a.p.zijlstra@chello.nl>, Wu Fengguang <fengguang.wu@intel.com>, Salman Qazi <sqazi@google.com>, linux-kernel@vger.kernel.org, linux-mm@kvack.org Subject: [PATCH] mm: remove worrying dead code from find_get_pages() Date: Wed, 23 Feb 2011 21:31:50 -0800 (PST) [thread overview] Message-ID: <alpine.LSU.2.00.1102232127590.2239@sister.anvils> (raw) The radix_tree_deref_retry() case in find_get_pages() has a strange little excrescence, not seen in the other gang lookups: it looks like the start of an abandoned attempt to guarantee forward progress in a case that cannot arise. ret should always be 0 here: if it isn't, then going back to restart will leak references to pages already gotten. There used to be a comment saying nr_found is necessarily 1 here: that's not quite true, but the radix_tree_deref_retry() case is peculiar to the entry at index 0, when we race with it being moved out of the radix_tree root or back. Remove the worrisome two lines, add a brief comment here and in find_get_pages_contig() and find_get_pages_tag(), and a WARN_ON in find_get_pages() should it ever be seen elsewhere than at 0. Signed-off-by: Hugh Dickins <hughd@google.com> --- mm/filemap.c | 18 ++++++++++++++++-- 1 file changed, 16 insertions(+), 2 deletions(-) --- 2.6.38-rc6/mm/filemap.c 2011-01-18 22:04:56.000000000 -0800 +++ linux/mm/filemap.c 2011-02-23 15:45:47.000000000 -0800 @@ -782,9 +782,13 @@ repeat: page = radix_tree_deref_slot((void **)pages[i]); if (unlikely(!page)) continue; + + /* + * This can only trigger when the entry at index 0 moves out + * of or back to the root: none yet gotten, safe to restart. + */ if (radix_tree_deref_retry(page)) { - if (ret) - start = pages[ret-1]->index; + WARN_ON(start | i); goto restart; } @@ -834,6 +838,11 @@ repeat: page = radix_tree_deref_slot((void **)pages[i]); if (unlikely(!page)) continue; + + /* + * This can only trigger when the entry at index 0 moves out + * of or back to the root: none yet gotten, safe to restart. + */ if (radix_tree_deref_retry(page)) goto restart; @@ -894,6 +903,11 @@ repeat: page = radix_tree_deref_slot((void **)pages[i]); if (unlikely(!page)) continue; + + /* + * This can only trigger when the entry at index 0 moves out + * of or back to the root: none yet gotten, safe to restart. + */ if (radix_tree_deref_retry(page)) goto restart;
WARNING: multiple messages have this Message-ID (diff)
From: Hugh Dickins <hughd@google.com> To: Nick Piggin <npiggin@kernel.dk> Cc: Andrew Morton <akpm@linux-foundation.org>, Peter Zijlstra <a.p.zijlstra@chello.nl>, Wu Fengguang <fengguang.wu@intel.com>, Salman Qazi <sqazi@google.com>, linux-kernel@vger.kernel.org, linux-mm@kvack.org Subject: [PATCH] mm: remove worrying dead code from find_get_pages() Date: Wed, 23 Feb 2011 21:31:50 -0800 (PST) [thread overview] Message-ID: <alpine.LSU.2.00.1102232127590.2239@sister.anvils> (raw) The radix_tree_deref_retry() case in find_get_pages() has a strange little excrescence, not seen in the other gang lookups: it looks like the start of an abandoned attempt to guarantee forward progress in a case that cannot arise. ret should always be 0 here: if it isn't, then going back to restart will leak references to pages already gotten. There used to be a comment saying nr_found is necessarily 1 here: that's not quite true, but the radix_tree_deref_retry() case is peculiar to the entry at index 0, when we race with it being moved out of the radix_tree root or back. Remove the worrisome two lines, add a brief comment here and in find_get_pages_contig() and find_get_pages_tag(), and a WARN_ON in find_get_pages() should it ever be seen elsewhere than at 0. Signed-off-by: Hugh Dickins <hughd@google.com> --- mm/filemap.c | 18 ++++++++++++++++-- 1 file changed, 16 insertions(+), 2 deletions(-) --- 2.6.38-rc6/mm/filemap.c 2011-01-18 22:04:56.000000000 -0800 +++ linux/mm/filemap.c 2011-02-23 15:45:47.000000000 -0800 @@ -782,9 +782,13 @@ repeat: page = radix_tree_deref_slot((void **)pages[i]); if (unlikely(!page)) continue; + + /* + * This can only trigger when the entry at index 0 moves out + * of or back to the root: none yet gotten, safe to restart. + */ if (radix_tree_deref_retry(page)) { - if (ret) - start = pages[ret-1]->index; + WARN_ON(start | i); goto restart; } @@ -834,6 +838,11 @@ repeat: page = radix_tree_deref_slot((void **)pages[i]); if (unlikely(!page)) continue; + + /* + * This can only trigger when the entry at index 0 moves out + * of or back to the root: none yet gotten, safe to restart. + */ if (radix_tree_deref_retry(page)) goto restart; @@ -894,6 +903,11 @@ repeat: page = radix_tree_deref_slot((void **)pages[i]); if (unlikely(!page)) continue; + + /* + * This can only trigger when the entry at index 0 moves out + * of or back to the root: none yet gotten, safe to restart. + */ if (radix_tree_deref_retry(page)) goto restart; -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Fight unfair telecom internet charges in Canada: sign http://stopthemeter.ca/ Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>
next reply other threads:[~2011-02-24 5:32 UTC|newest] Thread overview: 4+ messages / expand[flat|nested] mbox.gz Atom feed top 2011-02-24 5:31 Hugh Dickins [this message] 2011-02-24 5:31 ` [PATCH] mm: remove worrying dead code from find_get_pages() Hugh Dickins 2011-02-24 12:38 ` Peter Zijlstra 2011-02-24 12:38 ` Peter Zijlstra
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