Hi Andrew,
Any chance to take a look?
On Mon, Mar 02, 2020 at 04:26:38PM -0800, Minchan Kim wrote:
> 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.
>
> 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
>
