From mboxrd@z Thu Jan 1 00:00:00 1970 Content-Type: multipart/mixed; boundary="===============2858850245074653579==" MIME-Version: 1.0 From: Vlastimil Babka To: lkp@lists.01.org Subject: Re: [mm/sl[au]b] 3c4cafa313: canonical_address#:#[##] Date: Fri, 09 Sep 2022 23:16:51 +0200 Message-ID: In-Reply-To: List-Id: --===============2858850245074653579== Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: quoted-printable On 9/9/22 16:32, Hyeonggon Yoo wrote: > On Fri, Sep 09, 2022 at 03:44:19PM +0200, Vlastimil Babka wrote: >> On 9/9/22 13:05, Hyeonggon Yoo wrote: >> >> ----8<---- >> >> From d6f9fbb33b908eb8162cc1f6ce7f7c970d0f285f Mon Sep 17 00:00:00 2001 >> >> From: Vlastimil Babka >> >> Date: Fri, 9 Sep 2022 12:03:10 +0200 >> >> Subject: [PATCH 2/3] mm/migrate: make isolate_movable_page() skip sla= b pages >> >> = >> >> In the next commit we want to rearrange struct slab fields to allow a >> >> larger rcu_head. Afterwards, the page->mapping field will overlap >> >> with SLUB's "struct list_head slab_list", where the value of prev >> >> pointer can become LIST_POISON2, which is 0x122 + POISON_POINTER_DELT= A. >> >> Unfortunately the bit 1 being set can confuse PageMovable() to be a >> >> false positive and cause a GPF as reported by lkp [1]. >> >> = >> >> To fix this, make isolate_movable_page() skip pages with the PageSlab >> >> flag set. This is a bit tricky as we need to add memory barriers to S= LAB >> >> and SLUB's page allocation and freeing, and their counterparts to >> >> isolate_movable_page(). >> > = >> > Hello, I just took a quick grasp, >> > Is this approach okay with folio_test_anon()? >> = >> Not if used on a completely random page as compaction scanners can, but >> relies on those being first tested for PageLRU or coming from a page tab= le >> lookup etc. >> Not ideal huh. Well I could improve also by switching 'next' and 'slabs' >> field and relying on the fact that the value of LIST_POISON2 doesn't inc= lude >> 0x1, just 0x2. > = > What about swapping counters and freelist? > freelist should be always aligned. = Great suggestion, thanks! Had to deal with SLAB too as there was list_head.prev also aliasing page->mapping. Wanted to use freelist as well, but turns out it's not aligned, so had to use s_mem instead. The patch that isolate_movable_page() skip slab pages was thus dropped. The result is in slab.git below and if nothing blows up, will restore it to -ne= xt https://git.kernel.org/pub/scm/linux/kernel/git/vbabka/slab.git/log/?h=3Dfo= r-6.1/fit_rcu_head --===============2858850245074653579==--