On Thu, Aug 29, 2024 at 10:24 PM Vlastimil Babka [off-list ref] wrote:

On 8/27/24 09:50, Barry Song wrote:

quoted

On Tue, Aug 27, 2024 at 7:38 PM Vlastimil Babka [off-list ref] wrote:

quoted

Ugh, wasn't aware, well spotted. So it means there at least shouldn't be
existing users of __GFP_NOFAIL with order > 1 :)

But also the check is in the hotpath, even before trying the pcplists, so we
could move it to __alloc_pages_slowpath() while extending it?

Agreed. I don't think it is reasonable to check the order and flags in
two different places especially rmqueue() has already had
gfp_flags & __GFP_NOFAIL operation and order > 1
overhead.

We can at least extend the current check to make some improvement
though I still believe Michal's suggestion of implementing OOPS_ON is a
better approach to pursue, as it doesn't crash the entire system
while ensuring the problematic process is terminated.

Linus made clear it's not a mm concern. If e.g. hardening people want to
pursuit that instead, they can.

BTW I think BUG_ON already works like this, if possible only the calling
process is terminated. panic happens in case of being in a irq context, or

you are right. This is a detail I overlooked in the last discussion.
BUG_ON has already been exactly the case to only terminate the bad
process if it can
(panic_on_oops=N and not in irq context).

due to panic_on_oops. Which the security people are setting to 1 anyway and
OOPS_ON would have to observe it too. So AFAICS the only difference from
BUG_ON would be not panic in the irq context, if panic_on_oops isn't set.

right.

(as for "no mm locks held" I think it's already satisfied at the points we
check for __GFP_NOFAIL).

Let me summarize the discussion:

Patch 1/4, which fixes the misuse of combining gfp_nofail and atomic
in vdpa driver, is necessary.
Patch 2/4, which updates the documentation to clarify that
non-blockable gfp_nofail is not
                  supported, is needed.
Patch 3/4: We will replace BUG_ON with WARN_ON_ONCE to warn when the
size is too large,
                 where gfp_nofail will return NULL.
Patch 4/4: We will move the order > 1 check from the current fast path
to the slow path and extend
                 the check of gfp_direct_reclaim flag also in the slow path.

If nobody has an objection, I will prepare v4 as above.

Thanks
Barry