On Thu, Mar 11, 2021 at 12:09:15PM +0100, Michal Hocko wrote: > Sorry for being dense but I do not follow. You have provided the > following example > spin_lock(&A); > <IRQ> > spin_lock(&A); > > if A == hugetlb_lock then we should never reenter with > free_huge_page What I'm saying is that if irq_disabled(), the that interrupt cannot happen, so the second spin_lock cannot happen, so the deadlock cannot happen. So: '!irqs_disabled() && in_atomic()' is sufficient to avoid the IRQ recursion deadlock. Also, Linus hates constructs like this: https://lkml.kernel.org/r/CAHk-=wht7kAeyR5xEW2ORj7m0hibVxZ3t+2ie8vNHLQfdbN2_g@xxxxxxxxxxxxxx > From the code simplicity POV (and hugetlb has grown a lot of complexity) > it would be really easiest to make sure __free_huge_page to be called > from a non-atomic process context. There are few ways to do that > - defer each call to a WQ - user visible which sucks > - defer from atomic or otherwise non-sleeping contextx - requires > reliable in_atomic AFAICS > - defer sleeping operations - makes the code flow more complex and it > would be again user visible in some cases. > > So I would say we are in "pick your own poison" kind of situation. Just to be clear: NAK on this patch and any and all ductape crap. Fix it properly, make hugetlb_lock, spool->lock IRQ-safe, move the workqueue into the CMA thing. The code really doesn't look _that_ complicated.
