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 >
