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[34.123.190.156]) by smtp.gmail.com with ESMTPSA id f7-20020a6bdd07000000b007922dc27aa6sm2677118ioc.24.2023.08.28.11.32.40 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 28 Aug 2023 11:32:41 -0700 (PDT) Date: Mon, 28 Aug 2023 18:32:40 +0000 From: Joel Fernandes To: Lorenzo Stoakes Cc: linux-kernel@vger.kernel.org, linux-kselftest@vger.kernel.org, linux-mm@kvack.org, Shuah Khan , Vlastimil Babka , Michal Hocko , Linus Torvalds , Kirill A Shutemov , "Liam R. Howlett" , "Paul E. McKenney" , Suren Baghdasaryan , Kalesh Singh , Lokesh Gidra Subject: Re: [PATCH v5 2/7] mm/mremap: Allow moves within the same VMA Message-ID: <20230828183240.GA1621761@google.com> References: <20230822015501.791637-1-joel@joelfernandes.org> <20230822015501.791637-3-joel@joelfernandes.org> <46196ba1-c54d-4c1d-954f-a0006602af99@lucifer.local> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <46196ba1-c54d-4c1d-954f-a0006602af99@lucifer.local> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Sun, Aug 27, 2023 at 10:21:14AM +0100, Lorenzo Stoakes wrote: [..] > > > > /* > > * Flags used by change_protection(). For now we make it a bitmap so > > diff --git a/mm/mremap.c b/mm/mremap.c > > index 035fbf542a8f..06baa13bd2c8 100644 > > --- a/mm/mremap.c > > +++ b/mm/mremap.c > > @@ -490,12 +490,13 @@ static bool move_pgt_entry(enum pgt_entry entry, struct vm_area_struct *vma, > > } > > > > /* > > - * A helper to check if a previous mapping exists. Required for > > - * move_page_tables() and realign_addr() to determine if a previous mapping > > - * exists before we can do realignment optimizations. > > + * A helper to check if aligning down is OK. The aligned address should fall > > + * on *no mapping*. For the stack moving down, that's a special move within > > + * the VMA that is created to span the source and destination of the move, > > + * so we make an exception for it. > > */ > > static bool can_align_down(struct vm_area_struct *vma, unsigned long addr_to_align, > > - unsigned long mask) > > + unsigned long mask, bool for_stack) > > { > > unsigned long addr_masked = addr_to_align & mask; > > > > @@ -504,7 +505,7 @@ static bool can_align_down(struct vm_area_struct *vma, unsigned long addr_to_ali > > * of the corresponding VMA, we can't align down or we will destroy part > > * of the current mapping. > > */ > > - if (vma->vm_start != addr_to_align) > > + if (!for_stack && vma->vm_start != addr_to_align) > > return false; > > I'm a little confused by this exception, is it very specifically for the > shift_arg_pages() case where can assume we are safe to just discard the > lower portion of the stack? > > Wouldn't the find_vma_intersection() line below fail in this case? I may be > missing something here :) I think you are right. In v4, this was not an issue as we did this: + if (!for_stack && vma->vm_start != addr_to_align) + return false; + + cur = find_vma_prev(vma->vm_mm, vma->vm_start, &prev); + if (WARN_ON_ONCE(cur != vma)) + return false; Which essentially means this patch is a NOOP in v5 for the stack case. So what we really want is the VMA previous to @vma and whether than subsumes the masked address. Should I just change it back to the v4 version then as above for both patch 1 and 2 and carry your review tags? This is also hard to test as it requires triggering the execve stack move case. Though it is not a bug (as it is essentially a NOOP), it still would be nice to test it. This is complicated by also the fact that mremap(2) itself does not allow overlapping moves. I could try to hardcode the unfavorable situation as I have done in the past to force that mremap warning. thanks, - Joel