On 19.04.22 10:08, Alistair Popple wrote: > David Hildenbrand <david@xxxxxxxxxx> writes: > >> On 19.04.22 09:29, Miaohe Lin wrote: >>> On 2022/4/19 11:51, Alistair Popple wrote: >>>> Miaohe Lin <linmiaohe@xxxxxxxxxx> writes: >>>> >>>>> There is a bug in unuse_pte(): when swap page happens to be unreadable, >>>>> page filled with random data is mapped into user address space. In case >>>>> of error, a special swap entry indicating swap read fails is set to the >>>>> page table. So the swapcache page can be freed and the user won't end up >>>>> with a permanently mounted swap because a sector is bad. And if the page >>>>> is accessed later, the user process will be killed so that corrupted data >>>>> is never consumed. On the other hand, if the page is never accessed, the >>>>> user won't even notice it. >>>> >>>> Hi Miaohe, >>>>> It seems we're not actually using the pfn that gets stored in the special swap >>>> entry here. Is my understanding correct? If so I think it would be better to use >>> >>> Yes, you're right. The pfn is not used now. What we need here is a special swap entry >>> to do the right things. I think we can change to store some debugging information instead >>> of pfn if needed in the future. >>> >>>> the new PTE markers Peter introduced[1] rather than adding another swap entry >>>> type. >>> >>> IIUC, we should not reuse that swap entry here. From definition: >>> >>> PTE markers >>> `=========' >>> ... >>> PTE marker is a new type of swap entry that is ony applicable to file >>> backed memories like shmem and hugetlbfs. It's used to persist some >>> pte-level information even if the original present ptes in pgtable are >>> zapped. >>> >>> It's designed for file backed memories while swapin error entry is for anonymous >>> memories. And there has some differences in processing. So it's not a good idea >>> to reuse pte markers. Or am I miss something? >> >> I tend to agree. As raised in my other reply, maybe we can simply reuse >> hwpoison entries and update the documentation of them accordingly. > > Unless I've missed something I don't think PTE markers should be restricted > solely to file backed memory. It's true that the only user of them at the moment > is UFFD-WP for file backed memory, but PTE markers are just a special swap entry > same as what is added here. There is a difference. What we want here is "there used to be something mapped but it's not readable anymore. Please fail hard when userspace tries accessing this.". Just like with hwpoison entries. What a pte marker expresses is that "here is nothing mapped right now but we have additional metadata available here. For file-backed memory, it translates to: If we ever touch this page, lookup the pagecache what to map here." In the anonymous memory world, this would map to "populate the zeropage or a fresh anonymous page on access." and keep the metadata around. Yes, one might argue that for uffd-wp on anonymous memory it might make sense to use pte marker as well, when no page has been populated yet. IIRC, trying to arm uffd-wp when there is nothing populated yet will simply get ignored. > > That said I don't think there has been any attempt to make PTE markers work for > anything other than UFFD-WP because it was unclear if there ever would be > another user. We discussed using it for softdirty tracking as well. -- Thanks, David / dhildenb
