On 2024/5/9 10:39, Gao Xiang wrote:
Hi,
On 2024/5/9 10:20, Barry Song wrote:
On Thu, May 9, 2024 at 12:58 AM <hailong.liu@xxxxxxxx> wrote:
From: "Hailong.Liu" <hailong.liu@xxxxxxxx>
Commit a421ef303008 ("mm: allow !GFP_KERNEL allocations for kvmalloc")
includes support for __GFP_NOFAIL, but it presents a conflict with
commit dd544141b9eb ("vmalloc: back off when the current task is
OOM-killed"). A possible scenario is as belows:
process-a
kvcalloc(n, m, GFP_KERNEL | __GFP_NOFAIL)
__vmalloc_node_range()
__vmalloc_area_node()
vm_area_alloc_pages()
--> oom-killer send SIGKILL to process-a
if (fatal_signal_pending(current)) break;
--> return NULL;
to fix this, do not check fatal_signal_pending() in vm_area_alloc_pages()
if __GFP_NOFAIL set.
Reported-by: Oven <liyangouwen1@xxxxxxxx>
Signed-off-by: Hailong.Liu <hailong.liu@xxxxxxxx>
---
mm/vmalloc.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/mm/vmalloc.c b/mm/vmalloc.c
index 6641be0ca80b..2f359d08bf8d 100644
--- a/mm/vmalloc.c
+++ b/mm/vmalloc.c
@@ -3560,7 +3560,7 @@ vm_area_alloc_pages(gfp_t gfp, int nid,
/* High-order pages or fallback path if "bulk" fails. */
while (nr_allocated < nr_pages) {
- if (fatal_signal_pending(current))
+ if (!(gfp & __GFP_NOFAIL) && fatal_signal_pending(current))
break;
why not !nofail ?
This seems a correct fix, but it undermines the assumption made in
commit dd544141b9eb
("vmalloc: back off when the current task is OOM-killed")
"
This may trigger some hidden problems, when caller does not handle
vmalloc failures, or when rollaback after failed vmalloc calls own
vmallocs inside. However all of these scenarios are incorrect: vmalloc
does not guarantee successful allocation, it has never been called with
__GFP_NOFAIL and threfore either should not be used for any rollbacks or
should handle such errors correctly and not lead to critical failures.
"
If a significant kvmalloc operation is performed with the NOFAIL flag, it risks
reverting the fix intended to address the OOM-killer issue in commit
dd544141b9eb.
Should we indeed permit the NOFAIL flag for large kvmalloc allocations?
Just from my perspective, I don't really care about kmalloc, vmalloc
or kvmalloc (__GFP_NOFAIL). I even don't care if it returns three
order-0 pages or a high-order page. I just would like to need a
virtual consecutive buffer (even it works slowly.) with __GFP_NOFAIL.
Because in some cases, writing fallback code may be tough and hard to
test if such fallback path is correct since it only triggers in extreme
workloads, and even such buffers are just used in a very short lifetime.
add some words...
^ here extreme cases were mostly just generated by syzkaller fuzzing
tests, but if real users try to use some configuration to compress more,
I still think it needs to be handled (even such kvmalloc may be slow if
falling back to order-0 allocations due to memory pressures)
Also see other FS discussion of __GFP_NOFAIL, e.g.
https://lore.kernel.org/all/ZcUQfzfQ9R8X0s47@tiehlicka/
In the worst cases, it usually just needs < 5 order-0 pages (for many
cases it only needs one page), but with kmalloc it will trigger WARN
if it occurs to > order-1 allocation. as I mentioned before.
With my limited understanding I don't see why it could any problem with
kvmalloc(__GFP_NOFAIL) since it has no difference of kmalloc(GFP_NOFAIL)
with order-0 allocation.
.. kvmalloc with order-0 pages to form a virtual consecutive buffer
just like several kmalloc(__GFP_NOFAIL) allocations together in the
callers, I don't see any difference of memory pressure here.
Thanks,
Gao Xiang
Thanks,
Gao XIang
