On Tue, 2012-12-25 at 02:28 +0100, Andrea Arcangeli wrote: > Hi everyone, > > On Mon, Dec 24, 2012 at 11:21:02AM -0800, Linus Torvalds wrote: > > On Mon, Dec 24, 2012 at 10:53 AM, Christoffer Dall > > <cdall@xxxxxxxxxxxxxxx> wrote: > > > > > > I think I may have found an issue with the PageHead macro, which > > > returns true for tail compound pages when CONFIG_PAGEFLAGS_EXTENDED is > > > not defined. > > > > Hmm. Your patch *looks* obviously correct, in that it actually makes > > the code match the comment just above it. And making PageHead() test > > just the "compound" flag (and thus a tail-page would trigger it too) > > sounds wrong. But I join you in the "let's check the expected > > semantics with the people who use it" chorus. > > Yes, it's wrong if PageHead returns true on a tail page. PageHead and > PageTail are mutually exclusive flags. Only PageCompound returns true > for both PageHead and PageTail. > > > The fact that it fixes a problem on KVM/ARM is obviously another good sign. > > > > At the same time, I wonder why it hasn't shown up as a problem on > > x86-32. On x86-64 PAGEFLAGS_EXTENDED is always true, but afaik, it > > should be possible to trigger this on 32-bit architectures if you just > > have SPARSEMEM && !SPARSEMEM_VMEMMAP. > > Most of the PageHead checks are consistently run on real head pages, > so they're unlikely to run on tail pages. When !PageHead is used in > the bugchecks, the bug would lead to a false negative in the worst > case. This may be why this didn't show up on x86 32bit? > > But AFIK no binary x86 kernel was shipped with THP compiled in, so > it's also hard to quantify the different configs for the x86 32bit > self-built kernel images out there. > > > And SPARSEMEM on x86-32 is enabled with NUMA or EXPERIMENTAL set. And > > afaik, x86-32 never has SPARSEMEM_VMEMMAP. So this should not be a > > very uncommon setup. > > > > Added Andrea and Kirill to the Cc, since most of the *uses* of > > PageHead() in the generic VM code are attributed to either of them > > according to "git blame". Left the rest of the email quoted for the > > new participants.. Also, you seem to have used Christoph's old SGI > > email address that I don't think is in use any more. > > > > Andrea? Kirill? Christoph? > > The fix looks good to me, thanks! > Andrea Hi Andrea, I have a question. The comment above PG_head_mask: * PG_reclaim is used in combination with PG_compound to mark the * head and tail of a compound page. This saves one page flag * but makes it impossible to use compound pages for the page cache. * The PG_reclaim bit would have to be used for reclaim or readahead * if compound pages enter the page cache. If hugetlbfs pages on x86_32 is not in page cache? > > -- > To unsubscribe, send a message with 'unsubscribe linux-mm' in > the body to majordomo@xxxxxxxxx. For more info on Linux MM, > see: http://www.linux-mm.org/ . > Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a> -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>