On 11/19/24 13:48, Pavel Tikhomirov wrote:
Hello,
I see that in kernel code we have couple of places where kvmalloc is
used with GFP_NOFS flag:
git grep kvmalloc.*NOFS
fs/bcachefs/journal_io.c: new_buf = kvmalloc(new_size, GFP_NOFS|
__GFP_NOWARN);
fs/ext4/xattr.c: buffer = kvmalloc(value_size, GFP_NOFS);
fs/f2fs/compress.c: cc->private = f2fs_kvmalloc(F2FS_I_SB(cc-
>inode), size, GFP_NOFS);
net/ceph/osdmap.c: state = kvmalloc(array_size(max,
sizeof(*state)), GFP_NOFS);
net/ceph/osdmap.c: weight = kvmalloc(array_size(max,
sizeof(*weight)), GFP_NOFS);
net/ceph/osdmap.c: addr = kvmalloc(array_size(max, sizeof(*addr)),
GFP_NOFS);
and with GFP_NOIO flag too:
git grep kvmalloc.*NOIO
drivers/md/dm-integrity.c: recalc_tags = kvmalloc(recalc_tags_size,
GFP_NOIO);
drivers/md/dm-ioctl.c: dmi = kvmalloc(param_kernel->data_size, GFP_NOIO
| __GFP_HIGH);
net/ceph/messenger_v2.c: buf = kvmalloc(len, GFP_NOIO);
net/ceph/osdmap.c: work = kvmalloc(work_size, GFP_NOIO);
And AFAIU documentation https://docs.kernel.org/core-api/gfp_mask-from-
fs-io.html#what-about-vmalloc-gfp-nofs vmalloc allocation with GFP_NOFS
may end up doing "GFP_KERNEL allocations deep inside the allocator",
which can potentially lead to deadlock in IO/FS code paths.
Does it mean that we should rework all those paths to
memalloc_noio_save / memalloc_noio_restore variant? Or is it already
safe to use kvmalloc(GFP_NOIO) in modern kernel?
Or maybe I misunderstand something, sorry in advance if that's the case.
Now when I've already sent a question I seemingly found the answer:
In commit 451769ebb7e79 ("mm/vmalloc: alloc GFP_NO{FS,IO} for vmalloc")
we add implicit memalloc_noXX_save/memalloc_noXX_restore at this code path:
+->kvmalloc
+-> ...
+-> __kvmalloc_node_noprof
+-> __vmalloc_node_range_noprof
+-> __vmalloc_area_node
So kvmalloc should be safe now with GFP_NOIO. Should we correct the
documentation?
--
Best regards, Tikhomirov Pavel
Senior Software Developer, Virtuozzo.
