[PATCH v2 2/2] mm: fix long time stall from mm_populate

[Minchan Kim]

Basically, fault handler releases mmap_sem before requesting readahead
and then it is supposed to retry lookup the page from page cache with
FAULT_FLAG_TRIED so that it avoids the live lock of infinite retry.

However, what happens if the fault handler find a page from page
cache and the page has readahead marker but are waiting under
writeback? Plus one more condition, it happens under mm_populate
which repeats faulting unless it encounters error. So let's assemble
conditions below.

       CPU 1                                                        CPU 2

- first loop
    mm_populate
     for ()
       ..
       ret = populate_vma_page_range
         __get_user_pages
           faultin_page
             handle_mm_fault
               filemap_fault
                 do_async_mmap_readahead
                   if (PageReadahead(pageA))
                     maybe_unlock_mmap_for_io
                       up_read(mmap_sem)
					                    shrink_page_list
                                                              pageout
                                                                SetPageReclaim(=SetPageReadahead)(pageA)
                                                                writepage
                                                                  SetPageWriteback(pageA)

                     page_cache_async_readahead()
		       ClearPageReadahead(pageA)
                 do_async_mmap_readahead
		 lock_page_maybe_drop_mmap
		   goto out_retry

					                    the pageA is reclaimed
							    and new pageB is populated to the file offset
							    and finally has become PG_readahead

- second loop

	  __get_user_pages
           faultin_page
             handle_mm_fault
               filemap_fault
                 do_async_mmap_readahead
                   if (PageReadahead(pageB))
                     maybe_unlock_mmap_for_io
                       up_read(mmap_sem)
					                    shrink_page_list
                                                              pageout
                                                                SetPageReclaim(=SetPageReadahead)(pageB)
                                                                writepage
                                                                  SetPageWriteback(pageB)

                     page_cache_async_readahead()
		       ClearPageReadahead(pageB)
                 do_async_mmap_readahead
		 lock_page_maybe_drop_mmap
		   goto out_retry

It could be repeated forever so it's livelock. without involving reclaim,
it could happens if ra_pages become zero by fadvise/other threads who
have same fd one doing randome while the other one is sequential
because page_cache_async_readahead has following condition check like
PageWriteback and ra_pages are never synchrnized with fadvise and
shrink_readahead_size_eio from other threads.

void page_cache_async_readahead(struct address_space *mapping,
                           unsigned long req_size)
{
        /* no read-ahead */
        if (!ra->ra_pages)
                return;

Thus, we need to limit fault retry from mm_populate like page
fault handler.

Fixes: 6b4c9f446981 ("filemap: drop the mmap_sem for all blocking operations")
Reviewed-by: Jan Kara <jack@xxxxxxx>
Signed-off-by: Minchan Kim <minchan@xxxxxxxxxx>
---
 mm/gup.c | 11 ++++++++---
 1 file changed, 8 insertions(+), 3 deletions(-)

diff --git a/mm/gup.c b/mm/gup.c
index 1b521e0ac1de..6f6548c63ad5 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -1133,7 +1133,7 @@ static __always_inline long __get_user_pages_locked(struct task_struct *tsk,
  *
  * This takes care of mlocking the pages too if VM_LOCKED is set.
  *
- * return 0 on success, negative error code on error.
+ * return number of pages pinned on success, negative error code on error.
  *
  * vma->vm_mm->mmap_sem must be held.
  *
@@ -1196,6 +1196,7 @@ int __mm_populate(unsigned long start, unsigned long len, int ignore_errors)
 	struct vm_area_struct *vma = NULL;
 	int locked = 0;
 	long ret = 0;
+	bool tried = false;
 
 	end = start + len;
 
@@ -1226,14 +1227,18 @@ int __mm_populate(unsigned long start, unsigned long len, int ignore_errors)
 		 * double checks the vma flags, so that it won't mlock pages
 		 * if the vma was already munlocked.
 		 */
-		ret = populate_vma_page_range(vma, nstart, nend, &locked);
+		ret = populate_vma_page_range(vma, nstart, nend,
+						tried ? NULL : &locked);
 		if (ret < 0) {
 			if (ignore_errors) {
 				ret = 0;
 				continue;	/* continue at next VMA */
 			}
 			break;
-		}
+		} else if (ret == 0)
+			tried = true;
+		else
+			tried = false;
 		nend = nstart + ret * PAGE_SIZE;
 		ret = 0;
 	}
-- 
2.25.0.265.gbab2e86ba0-goog