On Tue, Mar 16, 2021 at 02:09:47PM +0000, Matthew Wilcox wrote:
> If we get a memory failure in the middle of a file THP, I think we handle
> it poorly.
>
> int memory_failure(unsigned long pfn, int flags)
> ...
> if (TestSetPageHWPoison(p)) {
> ...
> orig_head = hpage = compound_head(p);
> ...
> if (PageTransHuge(hpage)) {
> if (try_to_split_thp_page(p, "Memory Failure") < 0) {
> action_result(pfn, MF_MSG_UNSPLIT_THP, MF_IGNORED);
> return -EBUSY;
> }
>
> static int try_to_split_thp_page(struct page *page, const char *msg)
> {
> lock_page(page);
> if (!PageAnon(page) || unlikely(split_huge_page(page))) {
> unsigned long pfn = page_to_pfn(page);
>
> unlock_page(page);
> if (!PageAnon(page))
> pr_info("%s: %#lx: non anonymous thp\n", msg, pfn);
> else
> pr_info("%s: %#lx: thp split failed\n", msg, pfn);
> put_page(page);
> return -EBUSY;
>
> So (for some reason) we don't even try to split a file THP. But then,
> if we take a page fault on a file THP:
>
> static struct page *next_uptodate_page(struct page *page,
> ...
> if (PageHWPoison(page))
> goto skip;
> (... but we're only testing the head page here, which isn't necessarily
> the one which got the error ...)
>
> if (pmd_none(*vmf->pmd) && PageTransHuge(page)) {
> vm_fault_t ret = do_set_pmd(vmf, page);
>
> So we now map the PMD-sized page into userspace, even though it has a
> HWPoison in it.
>
> I think there are two things that we should be doing:
>
> 1. Attempt to split THPs which are file-backed. That makes most of this
> problem disappear because there won't be THPs with HWPoison, mostly.
+Naoya. Could you give more context here?
> 2. When the THP fails to split, use a spare page flag to indicate that
> the THP contains a HWPoison bit in one of its subpages. There are a
> lot of PF_SECOND flags available for this purpose.
>
> but I know almost nothing about the memory-failure subsystem and I'm
> still learning all the complexities of THPs, so it's entirely possible
> I've overlooked something important.
I wounder if it would be cleaner to switch PG_hwpoison to PF_HEAD: if
split failed we posion whole compound page. Yes, we will waste more
memory, but it makes it much cleaner for user: just check if the page is
poisoned.
--
Kirill A. Shutemov
