-----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1 On 02/10/2015 04:06 PM, Andrea Arcangeli wrote: > On Tue, Feb 10, 2015 at 12:47:37AM +0200, Ebru Akagunduz 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. >> >> The patch was tested with a program that allocates 800MB of >> memory, and performs interleaved reads and writes, in a pattern >> that causes some 2MB areas to first see read accesses, resulting >> in the zero pfn being mapped there. >> >> To simulate memory fragmentation at allocation time, I modified >> do_huge_pmd_anonymous_page to return VM_FAULT_FALLBACK for read >> faults. >> >> Without the patch, only %50 of the program was collapsed into THP >> and the percentage did not increase over time. >> >> With this patch after 10 minutes of waiting khugepaged had >> collapsed %89 of the program's memory. > > This is very good idea, associating it with the sysctl is sensible > here as collapsing zeropages would affect the memory footprint in > the same way as none ptes. > > __collapse_huge_page_copy however is likely screwing with the > refcounts of the zero page. Did you have DEBUG_VM=y enabled? If > yes you should get one warning that the zeropage refcount > underflowed that could confirm my concern: In __collapse_huge_page_copy, the zero pte takes the same path as pte_none, so I believe that part of the code is correct. > So in short I think __collapse_huge_page_copy and > release_pte_pages needs an additional case that complements the > already existing special You are right that release_pte_pages needs a special case too, in order to skip refcounting on the zero page. Ebru? - -- All rights reversed -----BEGIN PGP SIGNATURE----- Version: GnuPG v1 iQEcBAEBAgAGBQJU2r3fAAoJEM553pKExN6DJPAH/1TT9uzS0/1wRcN7gn/UP0rb TpkKzihDOeQgEPfGjd6wUgepU0iVhMX80qBCqk0wIAPgZLnt4IxSl24f09Sm38Cn zAV0mLySmoaYNisf+qieZ/NF/PDiUOrxGzWJzvm7Ymqq8Mh94qdgpsLy2I+EQioT RqwbYMMB2XvH3mWOzhQUfnyG5mJMmZtpVcrJ4MIVVq5a3x+Ry668ZT75oNegni5W Hfax6/8jf4Bjpxc9I/9FvZXzZr9m9yVcGHoCckdGxlnsSSgd60B9b+EYy6AlJpqS xYkGhKSL0iAAoXYkmrtFdLpdhU/eqhgLb0V2NxcimjrzNG/0LE8fGhb/0SmPXUU= =085q -----END PGP SIGNATURE----- -- 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>