On Wed, Jan 15, 2025 at 3:47 AM Vlastimil Babka <vbabka@xxxxxxx> wrote:
>
> On 1/15/25 03:17, Alexei Starovoitov wrote:
> > From: Alexei Starovoitov <ast@xxxxxxxxxx>
> >
> > Introduce free_pages_nolock() that can free pages without taking locks.
> > It relies on trylock and can be called from any context.
> > Since spin_trylock() cannot be used in RT from hard IRQ or NMI
> > it uses lockless link list to stash the pages which will be freed
> > by subsequent free_pages() from good context.
> >
> > Do not use llist unconditionally. BPF maps continuously
> > allocate/free, so we cannot unconditionally delay the freeing to
> > llist. When the memory becomes free make it available to the
> > kernel and BPF users right away if possible, and fallback to
> > llist as the last resort.
> >
> > Signed-off-by: Alexei Starovoitov <ast@xxxxxxxxxx>
>
> Acked-by: Vlastimil Babka <vbabka@xxxxxxx>
>
> With:
>
> > @@ -4853,6 +4905,17 @@ void __free_pages(struct page *page, unsigned int order)
> > }
> > EXPORT_SYMBOL(__free_pages);
> >
> > +/*
> > + * Can be called while holding raw_spin_lock or from IRQ and NMI,
> > + * but only for pages that came from try_alloc_pages():
> > + * order <= 3, !folio, etc
>
> I think order > 3 is fine, as !pcp_allowed_order() case is handled too?
try_alloc_page() has:
if (!pcp_allowed_order(order))
return NULL;
to make sure it tries pcp first.
bpf has no use for order > 1. Even 3 is overkill,
but it's kinda free to support order <= 3, so why not.
> And
> what does "!folio" mean?
That's what we discussed last year.
__free_pages() has all the extra stuff if (!head) and
support for dropping ref on the middle page.
!folio captures this more broadly.
> > + */
> > +void free_pages_nolock(struct page *page, unsigned int order)
> > +{
> > + if (put_page_testzero(page))
> > + __free_unref_page(page, order, FPI_TRYLOCK);
>
> Hmm this will reach reset_page_owner() and thus stackdepot so same mental
> note as for patch 1.
save_stack() has recursions protection already. So should be fine.
> > +}
> > +
> > void free_pages(unsigned long addr, unsigned int order)
> > {
> > if (addr != 0) {
>
