From c372473a545edff2fdbc002fc67c181e17c7557b Mon Sep 17 00:00:00 2001 From: Lorenzo Stoakes Date: Fri, 31 Jan 2025 12:31:53 +0000 Subject: mm: completely abstract unnecessary adj_start calculation The adj_start calculation has been a constant source of confusion in the VMA merge code. There are two cases to consider, one where we adjust the start of the vmg->middle VMA (i.e. the vmg->__adjust_middle_start merge flag is set), in which case adj_start is calculated as: (1) adj_start = vmg->end - vmg->middle->vm_start And the case where we adjust the start of the vmg->next VMA (i.e. the vmg->__adjust_next_start merge flag is set), in which case adj_start is calculated as: (2) adj_start = -(vmg->middle->vm_end - vmg->end) We apply (1) thusly: vmg->middle->vm_start = vmg->middle->vm_start + vmg->end - vmg->middle->vm_start Which simplifies to: vmg->middle->vm_start = vmg->end Similarly, we apply (2) as: vmg->next->vm_start = vmg->next->vm_start + -(vmg->middle->vm_end - vmg->end) Noting that for these VMAs to be mergeable vmg->middle->vm_end == vmg->next->vm_start and so this simplifies to: vmg->next->vm_start = vmg->next->vm_start + -(vmg->next->vm_start - vmg->end) Which simplifies to: vmg->next->vm_start = vmg->end Therefore in each case, we simply need to adjust the start of the VMA to vmg->end (!) and can do away with this adj_start calculation. The only caveat is that we must ensure we update the vm_pgoff field correctly. We therefore abstract this entire calculation to a new function vmg_adjust_set_range() which performs this calculation and sets the adjusted VMA's new range using the general vma_set_range() function. We also must update vma_adjust_trans_huge() which expects the now-abstracted adj_start parameter. It turns out this is wholly unnecessary. In vma_adjust_trans_huge() the relevant code is: if (adjust_next > 0) { struct vm_area_struct *next = find_vma(vma->vm_mm, vma->vm_end); unsigned long nstart = next->vm_start; nstart += adjust_next; split_huge_pmd_if_needed(next, nstart); } The only case where this is relevant is when vmg->__adjust_middle_start is specified (in which case adj_next would have been positive), i.e. the one in which the vma specified is vmg->prev and this the sought 'next' VMA would be vmg->middle. We can therefore eliminate the find_vma() invocation altogether and simply provide the vmg->middle VMA in this instance, or NULL otherwise. Again we have an adj_next offset calculation: next->vm_start + vmg->end - vmg->middle->vm_start Where next == vmg->middle this simplifies to vmg->end as previously demonstrated. Therefore nstart is equal to vmg->end, which is already passed to vma_adjust_trans_huge() via the 'end' parameter and so this code (rather delightfully) simplifies to: if (next) split_huge_pmd_if_needed(next, end); With these changes in place, it becomes silly for commit_merge() to return vmg->target, as it is always the same and threaded through vmg, so we finally change commit_merge() to return an error value once again. This patch has no change in functional behaviour. Link: https://lkml.kernel.org/r/7bce2cd4b5afb56211822835d145471280c3dccc.1738326519.git.lorenzo.stoakes@oracle.com Signed-off-by: Lorenzo Stoakes Reviewed-by: Vlastimil Babka Cc: Jann Horn Cc: Liam Howlett Signed-off-by: Andrew Morton --- include/linux/huge_mm.h | 4 ++-- 1 file changed, 2 insertions(+), 2 deletions(-) (limited to 'include/linux') diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h index 93e509b6c00e..e1bea54820ff 100644 --- a/include/linux/huge_mm.h +++ b/include/linux/huge_mm.h @@ -404,7 +404,7 @@ int madvise_collapse(struct vm_area_struct *vma, struct vm_area_struct **prev, unsigned long start, unsigned long end); void vma_adjust_trans_huge(struct vm_area_struct *vma, unsigned long start, - unsigned long end, long adjust_next); + unsigned long end, struct vm_area_struct *next); spinlock_t *__pmd_trans_huge_lock(pmd_t *pmd, struct vm_area_struct *vma); spinlock_t *__pud_trans_huge_lock(pud_t *pud, struct vm_area_struct *vma); @@ -571,7 +571,7 @@ static inline int madvise_collapse(struct vm_area_struct *vma, static inline void vma_adjust_trans_huge(struct vm_area_struct *vma, unsigned long start, unsigned long end, - long adjust_next) + struct vm_area_struct *next) { } static inline int is_swap_pmd(pmd_t pmd) -- cgit v1.2.3