On Mon, Dec 21, 2020 at 11:56:36AM -0800, Hugh Dickins wrote: > On Mon, 23 Nov 2020, Andrew Morton wrote: > > On Fri, 20 Nov 2020 17:55:22 -0800 syzbot <syzbot+e5a33e700b1dd0da20a2@xxxxxxxxxxxxxxxxxxxxxxxxx> wrote: > > > > > Hello, > > > > > > syzbot found the following issue on: > > > > > > HEAD commit: 03430750 Add linux-next specific files for 20201116 > > > git tree: linux-next > > > console output: https://syzkaller.appspot.com/x/log.txt?x=13f80e5e500000 > > > kernel config: https://syzkaller.appspot.com/x/.config?x=a1c4c3f27041fdb8 > > > dashboard link: https://syzkaller.appspot.com/bug?extid=e5a33e700b1dd0da20a2 > > > compiler: gcc (GCC) 10.1.0-syz 20200507 > > > syz repro: https://syzkaller.appspot.com/x/repro.syz?x=12f7bc5a500000 > > > C reproducer: https://syzkaller.appspot.com/x/repro.c?x=10934cf2500000 > > > > Alex, your series "per memcg lru lock" changed the vmscan code rather a > > lot. Could you please take a look at that reproducer? > > Andrew, I promised I'd take a look at this syzreport too (though I think > we're agreed by now that it has nothing to do with per-memcg lru_lock). > > I did try, but (unlike Alex) did not manage to get the reproducer to > reproduce it. No doubt I did not try hard enough: I did rather lose > interest after seeing that it appears to involve someone with > CAP_SYS_ADMIN doing an absurdly large ioctl(BLKFRASET) on /dev/nullb0 > ("Null test block driver" enabled via CONFIG_BLK_DEV_NULL_BLK=y: that I > did enable) and faulting from it: presumably triggering an absurd amount > of readahead. > > Cc'ing Matthew since he has a particular interest in readahead, and > might be inspired to make some small safe change that would fix this, > and benefit realistic cases too; but on the whole it didn't look worth > worrying about - or at least not by me. Oh, interesting. Thanks for looping me in, I hadn't looked at this one at all. Building on the debugging you did, this is the interesting part of the backtrace to me: > > > try_to_free_pages+0x29f/0x720 mm/vmscan.c:3264 > > > __perform_reclaim mm/page_alloc.c:4360 [inline] > > > __alloc_pages_direct_reclaim mm/page_alloc.c:4381 [inline] > > > __alloc_pages_slowpath.constprop.0+0x917/0x2510 mm/page_alloc.c:4785 > > > __alloc_pages_nodemask+0x5f0/0x730 mm/page_alloc.c:4995 > > > alloc_pages_current+0x191/0x2a0 mm/mempolicy.c:2271 > > > alloc_pages include/linux/gfp.h:547 [inline] > > > __page_cache_alloc mm/filemap.c:977 [inline] > > > __page_cache_alloc+0x2ce/0x360 mm/filemap.c:962 > > > page_cache_ra_unbounded+0x3a1/0x920 mm/readahead.c:216 > > > do_page_cache_ra+0xf9/0x140 mm/readahead.c:267 > > > do_sync_mmap_readahead mm/filemap.c:2721 [inline] > > > filemap_fault+0x19d0/0x2940 mm/filemap.c:2809 So ra_pages has been set to something ridiculously large, and as a result, we call do_page_cache_ra() asking to read more memory than is available in the machine. Funny thing, we actually have a function to prevent this kind of situation, and it's force_page_cache_ra(). So this might fix the problem. I only tested that it compiles. I'll be happy to write up a proper changelog and sign-off for it if it works ... it'd be good to get it some soak testing on a variety of different workloads; changing this stuff is enormously subtle. As a testament to that, I think Fengguang got it wrong in commit 2cbea1d3ab11 -- async_size should have been 3 * ra_pages / 4, not ra_pages / 4 (because we read-behind by half the range, so we're looking for a page fault to happen a quarter of the way behind this fault ...) This is partially Roman's fault, see commit 600e19afc5f8. diff --git a/mm/filemap.c b/mm/filemap.c index d5e7c2029d16..43fe0f0ae3bb 100644 --- a/mm/filemap.c +++ b/mm/filemap.c @@ -2632,7 +2632,7 @@ static struct file *do_sync_mmap_readahead(struct vm_fault *vmf) ra->size = ra->ra_pages; ra->async_size = ra->ra_pages / 4; ractl._index = ra->start; - do_page_cache_ra(&ractl, ra->size, ra->async_size); + force_page_cache_ra(&ractl, ra, ra->size); return fpin; } diff --git a/mm/internal.h b/mm/internal.h index c43ccdddb0f6..5664b4b91340 100644 --- a/mm/internal.h +++ b/mm/internal.h @@ -49,8 +49,6 @@ void unmap_page_range(struct mmu_gather *tlb, unsigned long addr, unsigned long end, struct zap_details *details); -void do_page_cache_ra(struct readahead_control *, unsigned long nr_to_read, - unsigned long lookahead_size); void force_page_cache_ra(struct readahead_control *, struct file_ra_state *, unsigned long nr); static inline void force_page_cache_readahead(struct address_space *mapping, diff --git a/mm/readahead.c b/mm/readahead.c index c5b0457415be..f344c894c26a 100644 --- a/mm/readahead.c +++ b/mm/readahead.c @@ -246,7 +246,7 @@ EXPORT_SYMBOL_GPL(page_cache_ra_unbounded); * behaviour which would occur if page allocations are causing VM writeback. * We really don't want to intermingle reads and writes like that. */ -void do_page_cache_ra(struct readahead_control *ractl, +static void do_page_cache_ra(struct readahead_control *ractl, unsigned long nr_to_read, unsigned long lookahead_size) { struct inode *inode = ractl->mapping->host;
