David Hildenbrand writes: > On 19.04.22 09:29, Miaohe Lin wrote: >> On 2022/4/19 11:51, Alistair Popple wrote: >>> Miaohe Lin 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. 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. But I agree re-using hwpoison entries is probably a better fit if possible. - Alistair