linux-kernel - Question about vmalloc(GFP

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Message-ID: <112f93f3-455a-4b89-94c9-d12844d972ef@virtuozzo.com>
Date: Tue, 19 Nov 2024 13:48:15 +0800
From: Pavel Tikhomirov <ptikhomirov@...tuozzo.com>
To: Michal Hocko <mhocko@...e.com>
Cc: Linux Memory Management List <linux-mm@...ck.org>,
 lkml <linux-kernel@...r.kernel.org>,
 Andrew Morton <akpm@...ux-foundation.org>
Subject: Question about vmalloc(GFP_NOFS)

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.

-- 
Best regards, Tikhomirov Pavel
Senior Software Developer, Virtuozzo.