mm/madvise: MADV_COLLAPSE return EAGAIN when page cannot be isolated

MADV_COLLAPSE is a best-effort request that attempts to set an actionable
errno value if the request cannot be fulfilled at the time.  EAGAIN should
be used to communicate that a resource was temporarily unavailable, but
that the user may try again immediately.

SCAN_DEL_PAGE_LRU is an internal result code used when a page cannot be
isolated from it's LRU list.  Since this, like SCAN_PAGE_LRU, is likely a
transitory state, make MADV_COLLAPSE return EAGAIN so that users know they
may reattempt the operation.

Another important scenario to consider is race with khugepaged.
khugepaged might isolate a page while MADV_COLLAPSE is interested in it.
Even though racing with khugepaged might mean that the memory has already
been collapsed, signalling an errno that is non-intrinsic to that memory
or arguments provided to madvise(2) lets the user know that future
attempts might (and in this case likely would) succeed, and avoids
false-negative assumptions by the user.

Link: https://lkml.kernel.org/r/20220922184651.1016461-1-zokeefe@google.com
Fixes: a5e1d213d329 ("mm/madvise: introduce MADV_COLLAPSE sync hugepage collapse")
Signed-off-by: Zach O'Keefe <zokeefe@google.com>
Cc: Axel Rasmussen <axelrasmussen@google.com>
Cc: Chris Kennelly <ckennelly@google.com>
Cc: David Hildenbrand <david@redhat.com>
Cc: David Rientjes <rientjes@google.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: James Houghton <jthoughton@google.com>
Cc: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Miaohe Lin <linmiaohe@huawei.com>
Cc: Minchan Kim <minchan@kernel.org>
Cc: Pasha Tatashin <pasha.tatashin@soleen.com>
Cc: Peter Xu <peterx@redhat.com>
Cc: Rongwei Wang <rongwei.wang@linux.alibaba.com>
Cc: SeongJae Park <sj@kernel.org>
Cc: Song Liu <songliubraving@fb.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: Yang Shi <shy828301@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index 40fd9f7b3ed3bcabb9a0e11ba47c4bad314de5c4..b3ebe90a66d995d4f3d9360db33f7100e618f868 100644 (file)
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -2372,6 +2372,7 @@ static int madvise_collapse_errno(enum scan_result r)
        /* Resource temporary unavailable - trying again might succeed */
        case SCAN_PAGE_LOCK:
        case SCAN_PAGE_LRU:
+       case SCAN_DEL_PAGE_LRU:
                return -EAGAIN;
        /*
         * Other: Trying again likely not to succeed / error intrinsic to
@@ -2454,6 +2455,7 @@ int madvise_collapse(struct vm_area_struct *vma, struct vm_area_struct **prev,
                case SCAN_PAGE_LOCK:
                case SCAN_PAGE_COMPOUND:
                case SCAN_PAGE_LRU:
+               case SCAN_DEL_PAGE_LRU:
                        last_fail = result;
                        break;
                default: