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Howlett" , Vlastimil Babka , Mike Rapoport , Suren Baghdasaryan , Michal Hocko , David Rientjes , Shakeel Butt , Andrew Morton , Jann Horn , Pedro Falcato , "Matthew Wilcox (Oracle)" , Jan Kara , Nhat Pham , Johannes Weiner , Zi Yan , Matthew Brost , Joshua Hahn , Byungchul Park , Gregory Price , Ying Huang , Usama Arif , Kairui Song , Balbir Singh , =?utf-8?B?SsOpcsO0bWU=?= Glisse , Kiryl Shutsemau , Yu-cheng Yu , Dave Hansen , Dennis Zhou , Tejun Heo , Christoph Lameter , Peng Fan , Josef Bacik , Chris Mason , linux-mm@kvack.org, linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org, "Borislav Petkov (AMD)" , Mark Brown , "Dennis Zhou (Facebook)" , Rick Edgecombe , stable@vger.kernel.org Subject: Re: [PATCH 08/13] mm/migrate_device: fix ksmd oops on a migrated KSM page Message-ID: References: <20261111-mason-migrate-et-al-fixes-v1-0-3d2454362e8d@kernel.org> <20261111-mason-migrate-et-al-fixes-v1-8-3d2454362e8d@kernel.org> Content-Type: text/plain; charset=us-ascii 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U2FsdGVkX1+dW4oTk7yfZAh+2M7sKPeVo6FAcOMBYJ+HeTYOQiSE6HeV6xiSIIr98bQFzdT68TiwyvTDrnE+4fYgCBoGxBBHhlWMUZYxWp0pwXj9Pr+cUxA0XXl0SpjgILYh03rf23ovzgD/jsCer6xVOzxfxS2BAVKH78TtaPWBieA31hZmgukbprjzcFh9foDPU3ljB1aLhiubc0x2TMO0tCghqlyNvz6zXipr6BJyz2peg0BdYCfmZ1yJ4zNrk8pKaU6bdLja+YBK+drJ/e9ZgriLLoL+iJjYq4v48FgpydUBfzNXzBDgyssJkKZflZPVgJANgsHTrUK6Rm4LOoWoERRihP9UImNFDZWQeYr4m9HarOngfvqFX+xDh/5jSwlaCfjEbN161roHP6gwVfo66QPv9r0e4H/Go3IEFhfHmemVj/e8UgY/uHdMFGT/QSkksoboXZfvlvGHC0vA5NaQVi7p3qWLN4Mz/ITKe0t0BWtHtgWfAwgvJdFQKXwLjadtYNH2r2ABqDLuY3B1OiVoyUpt9e1C0+FaNzgL2QE5mGcF7cwxvlguVPyyGKkgEF9/BULRJU1SbJgZ8frvuyZqFByNejErAsWLiDhyL0QCt6TMVBJdTs1sRkM/gmjUchkALTKhS/XusMc4If17+Nx2ZV0mY4B0nrxP54llkeBb1J2m3UL5giTItj/DQ1c1MZ7UQQteHYMrxuVtbxasSENARbU54G53zTypZX9gFFVlf3M4LKl9CZxNb5AiYmpfbjpVa0KC/hlVrQK3tvlhy58wW7kNpMZpmsoKJxBnkjvTI8Gapt+Oo00eAk1JgqLJQWfV5aOVua6IJh1uFbmI4nf5qMb6wEGf1GbAdn2i0nJ4PYwebfNLj/lESWCbNFoBNMOJz6FgTGiiZrNT6i/tL/wUcqN9KniHcrrlkGOL/EPjwuGdctkCTccoqEbE7a341smzONRkcr6vCtcFGQ6 NI3IR0sS 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reboot. A process that maps the page can also > > loop in page faults when it next accesses the page. > > > > The problem arises because KSM doesn't know not to try to look at device > > private memory that does not exist in the direct map. > > > > Disallow this from happening by explicitly testing for this scenario in > > __migrate_device_pages(). > > > > Fixes: a5430dda8a3a ("mm/migrate: support un-addressable ZONE_DEVICE page in migration") > > Cc: > > Signed-off-by: Chris Mason > > [ ljs: tweak commit message, comment ] > > Signed-off-by: Lorenzo Stoakes (ARM) > > --- > > mm/migrate_device.c | 9 +++++++++ > > 1 file changed, 9 insertions(+) > > > > diff --git a/mm/migrate_device.c b/mm/migrate_device.c > > index 45e571dbfb4c..7ebd02ec303b 100644 > > --- a/mm/migrate_device.c > > +++ b/mm/migrate_device.c > > @@ -1205,6 +1205,15 @@ static void __migrate_device_pages(unsigned long *src_pfns, > > } > > > > > > + /* > > + * KSM doesn't know not to try to read device private pages, so > > + * disallow the combination. > > + */ > > + if (folio_is_device_private(newfolio) && folio_test_ksm(folio)) { > > + src_pfns[i] &= ~MIGRATE_PFN_MIGRATE; > > + goto next; > > In general, LGTM. I was just wondering if we could reject that earlier. because > apparently we already allocated a destination folio :) You could do it in migrate_vma_collect_pmd(), but that would prevent migration of KSM pages to DEVICE_COHERENT pages. DEVICE_COHERENT pages are always in the direct map and are by definition fine to look at, so I think it's better just to do the check here. Reviewed-by: Alistair Popple > -- > Cheers, > > David