lists.openwall.net | lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening PHC | |
Open Source and information security mailing list archives
| ||
|
Date: Thu, 23 Sep 2021 16:48:41 -0700 (PDT) From: Hugh Dickins <hughd@...gle.com> To: Zi Yan <ziy@...dia.com> cc: Yang Shi <shy828301@...il.com>, "Kirill A. Shutemov" <kirill@...temov.name>, Hugh Dickins <hughd@...gle.com>, Matthew Wilcox <willy@...radead.org>, Kent Overstreet <kent.overstreet@...il.com>, Linux FS-devel Mailing List <linux-fsdevel@...r.kernel.org>, Linux Kernel Mailing List <linux-kernel@...r.kernel.org>, Linux MM <linux-mm@...ck.org>, Johannes Weiner <hannes@...xchg.org>, Linus Torvalds <torvalds@...ux-foundation.org>, Andrew Morton <akpm@...ux-foundation.org>, "Darrick J. Wong" <djwong@...nel.org>, Christoph Hellwig <hch@...radead.org>, David Howells <dhowells@...hat.com>, Mike Kravetz <mike.kravetz@...cle.com> Subject: Re: Mapcount of subpages On Thu, 23 Sep 2021, Zi Yan wrote: > On 23 Sep 2021, at 17:54, Yang Shi wrote: > > On Thu, Sep 23, 2021 at 2:10 PM Hugh Dickins <hughd@...gle.com> wrote: > >> > >> NR_FILE_MAPPED being used for /proc/meminfo's "Mapped:" and a couple > >> of other such stats files, and for a reclaim heuristic in mm/vmscan.c. > >> > >> Allow ourselves more slack in NR_FILE_MAPPED accounting (either count > >> each pte as if it mapped the whole THP, or don't count a THP's ptes > >> at all - you opted for the latter in the "Mlocked:" accounting), > >> and I suspect subpage _mapcount could be abandoned. > > > > AFAIK, partial THP unmap may need the _mapcount information of every > > subpage otherwise the deferred split can't know what subpages could be > > freed. I believe Yang Shi is right insofar as the decision on whether it's worth queuing for deferred split is being done based on those subpage _mapcounts. That is a use I had not considered, and I've given no thought to how important or not it is. > > Could we just scan page tables of a THP during deferred split process > instead? Deferred split is a slow path already, so maybe it can afford > the extra work. But unless I misunderstand, actually carrying out the deferred split already unmaps, uses migration entries, and remaps the remaining ptes: needing no help from subpage _mapcounts to do those, and free the rest. Hugh
Powered by blists - more mailing lists