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

[Alexandru Elisei <alexandru.elisei@arm.com>]

Hi,

On Tue, Nov 28, 2023 at 05:57:31PM +0100, David Hildenbrand wrote:
> 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.

Great, thank you for the feedback, I will use this approach for the next
iteration of the series.

Thanks,
Alex

> 
> -- 
> Cheers,
> 
> David / dhildenb
>