Re: [syzbot] [mm?] WARNING in ep_write_iter

On Sun, 16 Aug 2026 21:47:58 -0400 "Zi Yan" <[email protected]> wrote:

> >
> >> > I prefer Andrew's first suggestion.  If the user asks the kernel to copy 
> >> > too much data, just fail -- with no warning.
> >> 
> >> __GFP_WARN gets rid of all other warnings, even if user asks for a
> >> reasonable size. Why use such a big hammer?
> >
> > Because on many systems, WARN causes the kernel to crash.  You don't 
> > want the entire system to crash just because the user asked for more 
> > memory than was available.
> 
> User asking for more memory that what is available is pretty common and
> should not trigger a WARN or crash, unless you have panic_on_oom set.

I assume Alan is referring to panic_on_warn.

Heaven knows how common panic_on_warn usage is.  Gemini tells me "There
is no exact global headcount or precise user metric for how many people
use panic_on_warn.  However, the setting is widely enabled across a few
billion Android devices and many cloud/server provider host kernels
where automated failover makes a full reboot preferable to running with
an unknown warning state".

So I do think that WARNs are more serious than we (mm developers) tend
to assume.

So we just shouldn't permit userspace to trivially trigger a
page-allocation WARN.  Especially if the caller is perfectly capable of
handling an ENOMEM allocation failure, as appears to be the case with
usb-gadget.

(Does usb-gadget actually get used by Android?  Surely not by cloud
providers!)

(Can this WARN be triggered by unprivileged userspace?  I didn't look,
this matters a lot).

> You can mmap a virtual address range bigger than your physical memory
> size plus your swap space and try to fault all pages in. That would
> cause OOM and the system should not crash.

Right.  As long as it doesn't trigger a WARN!




Perhaps we should revisit this.

Why are we emitting a WARN if an allocation fails, given that this will
often panic the kernel?  Should we on the core MM side dial that back
to a pr_warn() and a helpful backtrace?