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Date: Fri, 8 Jan 2021 19:12:28 -0500 From: Andrea Arcangeli <aarcange@...hat.com> To: Linus Torvalds <torvalds@...ux-foundation.org> Cc: Will Deacon <will@...nel.org>, Linux-MM <linux-mm@...ck.org>, Linux Kernel Mailing List <linux-kernel@...r.kernel.org>, Yu Zhao <yuzhao@...gle.com>, Andy Lutomirski <luto@...nel.org>, Peter Xu <peterx@...hat.com>, Pavel Emelyanov <xemul@...nvz.org>, Mike Kravetz <mike.kravetz@...cle.com>, Mike Rapoport <rppt@...ux.vnet.ibm.com>, Minchan Kim <minchan@...nel.org>, Peter Zijlstra <peterz@...radead.org>, Hugh Dickins <hughd@...gle.com>, "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>, Matthew Wilcox <willy@...radead.org>, Oleg Nesterov <oleg@...hat.com>, Jann Horn <jannh@...gle.com>, Kees Cook <keescook@...omium.org>, John Hubbard <jhubbard@...dia.com>, Leon Romanovsky <leonro@...dia.com>, Jason Gunthorpe <jgg@...pe.ca>, Jan Kara <jack@...e.cz>, Kirill Tkhai <ktkhai@...tuozzo.com>, Nadav Amit <nadav.amit@...il.com>, Andrew Morton <akpm@...ux-foundation.org> Subject: Re: [PATCH 2/2] mm: soft_dirty: userfaultfd: introduce wrprotect_tlb_flush_pending On Fri, Jan 08, 2021 at 11:25:21AM -0800, Linus Torvalds wrote: > On Fri, Jan 8, 2021 at 9:53 AM Andrea Arcangeli <aarcange@...hat.com> wrote: > > > > Do you intend to eventually fix the zygote vmsplice case or not? > > Because in current upstream it's not fixed currently using the > > enterprise default config. > > Is this the hugepage case? Neither of your patches actually touched > that, so I've forgotten the details. The two patches only fixed the TLB flushing deferral in clear_refs and uffd-wp. So I didn't actually try to fix the hugepage case by adding the page_count checks there too. I could try to do that at least it'd be consistent but I still would try to find an alternate solution later. > > Irrelevant special case as in: long term GUP pin on the memory? > > Irrelevant special case in that > > (a) an extra COW shouldn't be a correctness issue unless somebody > does something horribly wrong (and obviously the code that hasn't > taken the mmap_lock for writing are then examples of that) > > and > > (b) it's not a performance issue either unless you can find a real > load that does it. > > Hmm? For b) I don't have an hard time to imagine `ps` hanging for seconds, if clear_refs is touched on a 4T mm, but b) is not the main concern. Having to rely on a) is the main concern and it's not about tlb flushes but the long term GUP pins. Thanks, Andrea
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