Messages in this thread Patch in this message | | | Date | Wed, 31 May 2017 02:29:20 -0700 | From | Omar Sandoval <> | Subject | Re: [lkp-robot] [btrfs] beeeccca9b: WARNING:at_mm/util.c:#kvmalloc_node |
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On Wed, May 31, 2017 at 11:19:05AM +0200, Michal Hocko wrote: > On Wed 31-05-17 02:12:02, Omar Sandoval wrote: > > On Wed, May 31, 2017 at 08:51:28AM +0200, Michal Hocko wrote: > > > On Wed 31-05-17 14:30:33, kernel test robot wrote: > > > > > > > > FYI, we noticed the following commit: > > > > > > > > commit: beeeccca9bebcec386cc31c250cff8a06cf27034 ("btrfs: Use kvzalloc instead of kzalloc/vmalloc in alloc_bitmap") > > > > https://git.kernel.org/cgit/linux/kernel/git/next/linux-next.git master > > > > > > I have intentionally skipped alloc_bitmap because it relies on GFP_NOFS. > > > This doesn't work properly when falling back to vmalloc and that is what > > > the warning reported here says. I believe the right approach is to check > > > whether the GFP_NOFS is _really_ needed and document why if yes. > > > Otherwise drop the NOFS part in one patch with the explanation and > > > convert it to kvmalloc in a separate patch. > > > > Unfortunately we really do need GFP_NOFS here, the free space tree is > > modified while we are committing a fs transaction, sometimes in the > > critical section when we block new operations from joining the > > transaction. > > OK, please document this. > > > Looking at the comment in kvmalloc_node(): > > > > /* > > * vmalloc uses GFP_KERNEL for some internal allocations (e.g page tables) > > * so the given set of flags has to be compatible. > > */ > > WARN_ON_ONCE((flags & GFP_KERNEL) != GFP_KERNEL); > > > > has alloc_bitmap() always been broken by virtue of calling vmalloc() > > with GFP_NOFS? > > yes. vmalloc is simply not GFP_NOFS safe as it performs GFP_KERNEL > hardcoded allocations. The way out of this is to use > memalloc_nofs_{save,restore} around kvmalloc call.
Ok, thanks. Something like this untested patch instead of the one in linux-next?
diff --git a/fs/btrfs/free-space-tree.c b/fs/btrfs/free-space-tree.c index fc0bd8406758..5abd3cd71144 100644 --- a/fs/btrfs/free-space-tree.c +++ b/fs/btrfs/free-space-tree.c @@ -153,21 +153,18 @@ static inline u32 free_space_bitmap_size(u64 size, u32 sectorsize) static u8 *alloc_bitmap(u32 bitmap_size) { - void *mem; + u8 *ret; + unsigned int nofs_flag; /* - * The allocation size varies, observed numbers were < 4K up to 16K. - * Using vmalloc unconditionally would be too heavy, we'll try - * contiguous allocations first. + * GFP_NOFS doesn't work with kvmalloc(), but we really can't recurse + * into the filesystem as the free space bitmap can be modified in the + * critical section of a transaction commit. */ - if (bitmap_size <= PAGE_SIZE) - return kzalloc(bitmap_size, GFP_NOFS); - - mem = kzalloc(bitmap_size, GFP_NOFS | __GFP_NOWARN); - if (mem) - return mem; - - return __vmalloc(bitmap_size, GFP_NOFS | __GFP_ZERO, PAGE_KERNEL); + nofs_flag = memalloc_nofs_save(); + ret = kvmalloc(bitmap_size, GFP_KERNEL); + memalloc_nofs_restore(nofs_flag); + return ret; } int convert_free_space_to_bitmaps(struct btrfs_trans_handle *trans, Dave, would you prefer to replace the patch we have now or do an incremental patch on top of it?
Michal, is there some reason we can't have kvmalloc() with !(gfp & __GFP_FS) just do the memalloc_nofs dance internally?
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