On Wed, 18 Feb 2015 19:08:12 -0500 Rik van Riel <riel@xxxxxxxxxx> wrote: > -----BEGIN PGP SIGNED MESSAGE----- > Hash: SHA1 > > On 02/18/2015 06:31 PM, Andrew Morton wrote: > > On Wed, 11 Feb 2015 23:03:55 +0200 Ebru Akagunduz > > <ebru.akagunduz@xxxxxxxxx> wrote: > > > >> This patch improves THP collapse rates, by allowing zero pages. > >> > >> Currently THP can collapse 4kB pages into a THP when there are up > >> to khugepaged_max_ptes_none pte_none ptes in a 2MB range. This > >> patch counts pte none and mapped zero pages with the same > >> variable. > > > > So if I'm understanding this correctly, with the default value of > > khugepaged_max_ptes_none (HPAGE_PMD_NR-1), if an application > > creates a 2MB area which contains 511 mappings of the zero page and > > one real page, the kernel will proceed to turn that area into a > > real, physical huge page. So it consumes 2MB of memory which would > > not have previously been allocated? > > This is equivalent to an application doing a write fault > to a 2MB area that was previously untouched, going into > do_huge_pmd_anonymous_page() and receiving a 2MB page. > > > If so, this might be rather undesirable behaviour in some > > situations (and ditto the current behaviour for pte_none ptes)? > > > > This can be tuned by adjusting khugepaged_max_ptes_none, > > The example of directly going into do_huge_pmd_anonymous_page() > is not influenced by the tunable. > > It may indeed be undesirable in some situations, but I am > not sure how to detect those... Here's a live one: https://bugzilla.kernel.org/show_bug.cgi?id=93111 Application does MADV_DONTNEED to free up a load of memory and then khugepaged comes along and pages that memory back in again. It seems a bit silly to do this after userspace has deliberately discarded those pages! Presumably MADV_NOHUGEPAGE can be used to prevent this, but it's a bit of a hand-grenade. I guess the MADV_DONTNEED manpage should be updated to explain all this? -- 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>