On 6/11/21 3:49 PM, Jann Horn wrote:
On Sat, Jun 12, 2021 at 12:36 AM Andrew Morton
<akpm@xxxxxxxxxxxxxxxxxxxx> wrote:
On Fri, 11 Jun 2021 18:15:45 +0200 Jann Horn <jannh@xxxxxxxxxx> wrote:
+/* Equivalent to calling put_page() @refs times. */
+static void put_page_refs(struct page *page, int refs)
+{
+ VM_BUG_ON_PAGE(page_ref_count(page) < refs, page);
I don't think there's a need to nuke the whole kernel in this case.
Can we warn then simply leak the page? That way we have a much better
chance of getting a good bug report.
Ah, yeah, I guess that makes sense. I had just copied this over from
put_compound_head(), and figured it was fine to keep it as-is, but I
guess changing it would be reasonable. I'm not quite sure what the
best way to do that would be though.
I guess the check should go away in !DEBUG_VM builds?
Should I just explicitly put the check in an ifdef block? Like so:
#ifdef CONFIG_DEBUG_VM
if (VM_WARN_ON_ONCE_PAGE(...))
return;
#endif
Or, since inline ifdeffery looks ugly, get rid of the explicit ifdef,
Agreed: VM_WARN_ON_ONCE_PAGE(), at least at the API level, seems like
the best thing to use here. However, as you point out below, it needs a
little something first.
and change the !DEBUG_VM definition of VM_WARN_ON_ONCE_PAGE() as
follows so that the branch is compiled away?
#define VM_WARN_ON_ONCE_PAGE(cond, page) (BUILD_BUG_ON_INVALID(cond), false)
That would look kinda neat, but it would be different from the
behavior of WARN_ON(), which still returns the original condition even
in !BUG builds, so that could be confusing...
The VM_WARN_ON_ONCE_PAGE() is not implemented exactly right
in the !CONFIG_DEBUG_VM case. IMHO it should follow the WARN*()
behavior, and return the original condition and keep going
in that case.
Then you could use it directly here.
thanks,
--
John Hubbard
NVIDIA
