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Howlett" , "Paul E. McKenney" , Suren Baghdasaryan , Kalesh Singh , Lokesh Gidra Subject: Re: [PATCH v6 2/7] mm/mremap: Allow moves within the same VMA for stack moves Message-ID: References: <20230903151328.2981432-1-joel@joelfernandes.org> <20230903151328.2981432-3-joel@joelfernandes.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20230903151328.2981432-3-joel@joelfernandes.org> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Sun, Sep 03, 2023 at 03:13:23PM +0000, Joel Fernandes (Google) wrote: > For the stack move happening in shift_arg_pages(), the move is happening > within the same VMA which spans the old and new ranges. > > In case the aligned address happens to fall within that VMA, allow such > moves and don't abort the mremap alignment optimization. > > In the regular non-stack mremap case, we cannot allow any such moves as > will end up destroying some part of the mapping (either the source of > the move, or part of the existing mapping). So just avoid it for stack > moves. > > Signed-off-by: Joel Fernandes (Google) > --- > fs/exec.c | 2 +- > include/linux/mm.h | 2 +- > mm/mremap.c | 33 +++++++++++++++++++-------------- > 3 files changed, 21 insertions(+), 16 deletions(-) > > diff --git a/fs/exec.c b/fs/exec.c > index 1a827d55ba94..244925307958 100644 > --- a/fs/exec.c > +++ b/fs/exec.c > @@ -712,7 +712,7 @@ static int shift_arg_pages(struct vm_area_struct *vma, unsigned long shift) > * process cleanup to remove whatever mess we made. > */ > if (length != move_page_tables(vma, old_start, > - vma, new_start, length, false)) > + vma, new_start, length, false, true)) > return -ENOMEM; > > lru_add_drain(); > diff --git a/include/linux/mm.h b/include/linux/mm.h > index 406ab9ea818f..e635d1fc73b6 100644 > --- a/include/linux/mm.h > +++ b/include/linux/mm.h > @@ -2458,7 +2458,7 @@ int get_cmdline(struct task_struct *task, char *buffer, int buflen); > extern unsigned long move_page_tables(struct vm_area_struct *vma, > unsigned long old_addr, struct vm_area_struct *new_vma, > unsigned long new_addr, unsigned long len, > - bool need_rmap_locks); > + bool need_rmap_locks, bool for_stack); > > /* > * Flags used by change_protection(). For now we make it a bitmap so > diff --git a/mm/mremap.c b/mm/mremap.c > index 1011326b7b80..2b51f8b7cad8 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; > > @@ -504,9 +505,13 @@ 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; > > + /* In the stack case we explicitly permit in-VMA alignment. */ > + if (for_stack && addr_masked >= vma->vm_start) > + return true; > + > /* > * Make sure the realignment doesn't cause the address to fall on an > * existing mapping. > @@ -517,7 +522,7 @@ static bool can_align_down(struct vm_area_struct *vma, unsigned long addr_to_ali > /* Opportunistically realign to specified boundary for faster copy. */ > static void try_realign_addr(unsigned long *old_addr, struct vm_area_struct *old_vma, > unsigned long *new_addr, struct vm_area_struct *new_vma, > - unsigned long mask) > + unsigned long mask, bool for_stack) > { > /* Skip if the addresses are already aligned. */ > if ((*old_addr & ~mask) == 0) > @@ -528,8 +533,8 @@ static void try_realign_addr(unsigned long *old_addr, struct vm_area_struct *old > return; > > /* Ensure realignment doesn't cause overlap with existing mappings. */ > - if (!can_align_down(old_vma, *old_addr, mask) || > - !can_align_down(new_vma, *new_addr, mask)) > + if (!can_align_down(old_vma, *old_addr, mask, for_stack) || > + !can_align_down(new_vma, *new_addr, mask, for_stack)) > return; > > *old_addr = *old_addr & mask; > @@ -539,7 +544,7 @@ static void try_realign_addr(unsigned long *old_addr, struct vm_area_struct *old > unsigned long move_page_tables(struct vm_area_struct *vma, > unsigned long old_addr, struct vm_area_struct *new_vma, > unsigned long new_addr, unsigned long len, > - bool need_rmap_locks) > + bool need_rmap_locks, bool for_stack) > { > unsigned long extent, old_end; > struct mmu_notifier_range range; > @@ -559,9 +564,9 @@ unsigned long move_page_tables(struct vm_area_struct *vma, > * If possible, realign addresses to PMD boundary for faster copy. > * Only realign if the mremap copying hits a PMD boundary. > */ > - if ((vma != new_vma) > - && (len >= PMD_SIZE - (old_addr & ~PMD_MASK))) > - try_realign_addr(&old_addr, vma, &new_addr, new_vma, PMD_MASK); > + if (len >= PMD_SIZE - (old_addr & ~PMD_MASK)) > + try_realign_addr(&old_addr, vma, &new_addr, new_vma, PMD_MASK, > + for_stack); > > flush_cache_range(vma, old_addr, old_end); > mmu_notifier_range_init(&range, MMU_NOTIFY_UNMAP, 0, vma->vm_mm, > @@ -708,7 +713,7 @@ static unsigned long move_vma(struct vm_area_struct *vma, > } > > moved_len = move_page_tables(vma, old_addr, new_vma, new_addr, old_len, > - need_rmap_locks); > + need_rmap_locks, false); > if (moved_len < old_len) { > err = -ENOMEM; > } else if (vma->vm_ops && vma->vm_ops->mremap) { > @@ -722,7 +727,7 @@ static unsigned long move_vma(struct vm_area_struct *vma, > * and then proceed to unmap new area instead of old. > */ > move_page_tables(new_vma, new_addr, vma, old_addr, moved_len, > - true); > + true, false); > vma = new_vma; > old_len = new_len; > old_addr = new_addr; > -- > 2.42.0.283.g2d96d420d3-goog > Looks good to me, thanks Reviewed-by: Lorenzo Stoakes