Re: [PATCH RFC v2 05/27] mm: page_alloc: Add an arch hook to allow prep_new_page() to fail

[David Hildenbrand]

On 27.11.23 13:09, Alexandru Elisei wrote:
> Hi,
>
> Thank you so much for your comments, there are genuinely useful.
>
> On Fri, Nov 24, 2023 at 08:35:47PM +0100, David Hildenbrand wrote:
> > On 19.11.23 17:56, Alexandru Elisei wrote:
> > > Introduce arch_prep_new_page(), which will be used by arm64 to reserve tag
> > > storage for an allocated page. Reserving tag storage can fail, for example,
> > > if the tag storage page has a short pin on it, so allow prep_new_page() ->
> > > arch_prep_new_page() to similarly fail.
> >
> > But what are the side-effects of this? How does the calling code recover?
> >
> > E.g., what if we need to populate a page into user space, but that
> > particular page we allocated fails to be prepared? So we inject a signal
> > into that poor process?
>
> When the page fails to be prepared, it is put back to the tail of the
> freelist with __free_one_page(.., FPI_TO_TAIL). If all the allocation paths
> are exhausted and no page has been found for which tag storage has been
> reserved, then that's treated like an OOM situation.
>
> I have been thinking about this, and I think I can simplify the code by
> making tag reservation a best effort approach. The page can be allocated
> even if reserving tag storage fails, but the page is marked as invalid in
> set_pte_at() (PAGE_NONE + an extra bit to tell arm64 that it needs tag
> storage) and next time it is accessed, arm64 will reserve tag storage in
> the fault handling code (the mechanism for that is implemented in patch #19
> of the series, "mm: mprotect: Introduce PAGE_FAULT_ON_ACCESS for
> mprotect(PROT_MTE)").
>
> With this new approach, prep_new_page() stays the way it is, and no further
> changes are required for the page allocator, as there are already arch
> callbacks that can be used for that, for example tag_clear_highpage() and
> arch_alloc_page(). The downside is extra page faults, which might impact
> performance.
>
> What do you think?

That sounds a lot more robust, compared to intermittent failures to 
allocate pages.

--
Cheers,

David / dhildenb