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[2003:d8:2f09:3f00:d228:bd67:7baa:d604]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-429c74ffb1esm1582035e9.5.2024.08.09.10.59.59 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Fri, 09 Aug 2024 11:00:00 -0700 (PDT) Message-ID: <8ef394e6-a964-41c4-b33c-0e940b6b9bd8@redhat.com> Date: Fri, 9 Aug 2024 19:59:58 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH 07/19] mm/fork: Accept huge pfnmap entries To: Peter Xu Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, Sean Christopherson , Oscar Salvador , Jason Gunthorpe , Axel Rasmussen , linux-arm-kernel@lists.infradead.org, x86@kernel.org, Will Deacon , Gavin Shan , Paolo Bonzini , Zi Yan , Andrew Morton , Catalin Marinas , Ingo Molnar , Alistair Popple , Borislav Petkov , Thomas Gleixner , kvm@vger.kernel.org, Dave Hansen , Alex Williamson , Yan Zhao References: <20240809160909.1023470-1-peterx@redhat.com> <20240809160909.1023470-8-peterx@redhat.com> From: David Hildenbrand Autocrypt: addr=david@redhat.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Rspamd-Server: rspam07 X-Rspamd-Queue-Id: C3BF514000B X-Stat-Signature: nepxcgsft6h3uiyzy3td4pgw4u5ko7nw X-Rspam-User: X-HE-Tag: 1723226404-294328 X-HE-Meta: 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 xP0zJ8F+ a+rRAXuClO3tuSXPriqIzkz4diFGu2CkvAXFSLWQgUNcvcZ1JFaC0aUuuDvWocf3X0+yBDz+HHHMyG2Xr+lJ4tr6VwuPhX+2kLjmynltjwwEXTCB7zrOoYzWWZZO/mvgJKcjwZG2hrk1AkQlFDUqru+wWan/PCLaqdxTSujDGHke081kjcqn194elllvHnnuczVP1RxvnhiRyTHa++tLLPgIfShGvd+KUWFNm5wJ+2I5xOJfx35cvdh0VUl6X30Mg5bC3uo9veNi1nd/14XgUFl4rB4hZyfL0SS0uzo554sWRW2NcLMUgdUaHqVGItKwTlU+6weGmqyUjFpmA6aFTVEj+QX7lVsxuH2vTGOurkOPqZa992oZFbh0BM2mLNhTMArf8CssUfQtMKOY= X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 09.08.24 19:15, Peter Xu wrote: > On Fri, Aug 09, 2024 at 06:32:44PM +0200, David Hildenbrand wrote: >> On 09.08.24 18:08, Peter Xu wrote: >>> Teach the fork code to properly copy pfnmaps for pmd/pud levels. Pud is >>> much easier, the write bit needs to be persisted though for writable and >>> shared pud mappings like PFNMAP ones, otherwise a follow up write in either >>> parent or child process will trigger a write fault. >>> >>> Do the same for pmd level. >>> >>> Signed-off-by: Peter Xu >>> --- >>> mm/huge_memory.c | 27 ++++++++++++++++++++++++--- >>> 1 file changed, 24 insertions(+), 3 deletions(-) >>> >>> diff --git a/mm/huge_memory.c b/mm/huge_memory.c >>> index 6568586b21ab..015c9468eed5 100644 >>> --- a/mm/huge_memory.c >>> +++ b/mm/huge_memory.c >>> @@ -1375,6 +1375,22 @@ int copy_huge_pmd(struct mm_struct *dst_mm, struct mm_struct *src_mm, >>> pgtable_t pgtable = NULL; >>> int ret = -ENOMEM; >>> + pmd = pmdp_get_lockless(src_pmd); >>> + if (unlikely(pmd_special(pmd))) { >>> + dst_ptl = pmd_lock(dst_mm, dst_pmd); >>> + src_ptl = pmd_lockptr(src_mm, src_pmd); >>> + spin_lock_nested(src_ptl, SINGLE_DEPTH_NESTING); >>> + /* >>> + * No need to recheck the pmd, it can't change with write >>> + * mmap lock held here. >>> + */ >>> + if (is_cow_mapping(src_vma->vm_flags) && pmd_write(pmd)) { >>> + pmdp_set_wrprotect(src_mm, addr, src_pmd); >>> + pmd = pmd_wrprotect(pmd); >>> + } >>> + goto set_pmd; >>> + } >>> + >> >> I strongly assume we should be using using vm_normal_page_pmd() instead of >> pmd_page() further below. pmd_special() should be mostly limited to GUP-fast >> and vm_normal_page_pmd(). > > One thing to mention that it has this: > > if (!vma_is_anonymous(dst_vma)) > return 0; Another obscure thing in this function. It's not the job of copy_huge_pmd() to make the decision whether to copy, it's the job of vma_needs_copy() in copy_page_range(). And now I have to suspect that uffd-wp is broken with this function, because as vma_needs_copy() clearly states, we must copy, and we don't do that for PMDs. Ugh. What a mess, we should just do what we do for PTEs and we will be fine ;) Also, we call copy_huge_pmd() only if "is_swap_pmd(*src_pmd) || pmd_trans_huge(*src_pmd) || pmd_devmap(*src_pmd)" Would that even be the case with PFNMAP? I suspect that pmd_trans_huge() would return "true" for special pfnmap, which is rather "surprising", but fortunate for us. Likely we should be calling copy_huge_pmd() if pmd_leaf() ... cleanup for another day. > > So it's only about anonymous below that. In that case I feel like the > pmd_page() is benign, and actually good. Yes, it would likely currently work. > > Though what you're saying here made me notice my above check doesn't seem > to be necessary, I mean, "(is_cow_mapping(src_vma->vm_flags) && > pmd_write(pmd))" can't be true when special bit is set, aka, pfnmaps.. and > if it's writable for CoW it means it's already an anon. > > I think I can probably drop that line there, perhaps with a > VM_WARN_ON_ONCE() making sure it won't happen. > >> >> Again, we should be doing this similar to how we handle PTEs. >> >> I'm a bit confused about the "unlikely(!pmd_trans_huge(pmd)" check, below: >> what else should we have here if it's not a migration entry but a present >> entry? > > I had a feeling that it was just a safety belt since the 1st day of thp > when Andrea worked that out, so that it'll work with e.g. file truncation > races. > > But with current code it looks like it's only anonymous indeed, so looks > not possible at least from that pov. Yes, as stated above, likely broken with UFFD-WP ... I really think we should make this code just behave like it would with PTEs, instead of throwing in more "different" handling. -- Cheers, David / dhildenb