From: Chinwen Chang <chinwen.chang@xxxxxxxxxxxx> Subject: mm: proc: smaps_rollup: do not stall write attempts on mmap_lock smaps_rollup will try to grab mmap_lock and go through the whole vma list until it finishes the iterating. When encountering large processes, the mmap_lock will be held for a longer time, which may block other write requests like mmap and munmap from progressing smoothly. There are upcoming mmap_lock optimizations like range-based locks, but the lock applied to smaps_rollup would be the coarse type, which doesn't avoid the occurrence of unpleasant contention. To solve aforementioned issue, we add a check which detects whether anyone wants to grab mmap_lock for write attempts. Link: http://lkml.kernel.org/r/1597715898-3854-4-git-send-email-chinwen.chang@xxxxxxxxxxxx Signed-off-by: Chinwen Chang <chinwen.chang@xxxxxxxxxxxx> Cc: Steven Price <steven.price@xxxxxxx> Cc: Michel Lespinasse <walken@xxxxxxxxxx> Cc: Matthias Brugger <matthias.bgg@xxxxxxxxx> Cc: Vlastimil Babka <vbabka@xxxxxxx> Cc: Daniel Jordan <daniel.m.jordan@xxxxxxxxxx> Cc: Davidlohr Bueso <dbueso@xxxxxxx> Cc: Chinwen Chang <chinwen.chang@xxxxxxxxxxxx> Cc: Alexey Dobriyan <adobriyan@xxxxxxxxx> Cc: "Matthew Wilcox (Oracle)" <willy@xxxxxxxxxxxxx> Cc: Jason Gunthorpe <jgg@xxxxxxxx> Cc: Song Liu <songliubraving@xxxxxx> Cc: Jimmy Assarsson <jimmyassarsson@xxxxxxxxx> Cc: Huang Ying <ying.huang@xxxxxxxxx> Cc: Daniel Kiss <daniel.kiss@xxxxxxx> Cc: Laurent Dufour <ldufour@xxxxxxxxxxxxx> Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> --- fs/proc/task_mmu.c | 66 ++++++++++++++++++++++++++++++++++++++++++- 1 file changed, 65 insertions(+), 1 deletion(-) --- a/fs/proc/task_mmu.c~mm-proc-smaps_rollup-do-not-stall-write-attempts-on-mmap_lock +++ a/fs/proc/task_mmu.c @@ -865,9 +865,73 @@ static int show_smaps_rollup(struct seq_ hold_task_mempolicy(priv); - for (vma = priv->mm->mmap; vma; vma = vma->vm_next) { + for (vma = priv->mm->mmap; vma;) { smap_gather_stats(vma, &mss, 0); last_vma_end = vma->vm_end; + + /* + * Release mmap_lock temporarily if someone wants to + * access it for write request. + */ + if (mmap_lock_is_contended(mm)) { + mmap_read_unlock(mm); + ret = mmap_read_lock_killable(mm); + if (ret) { + release_task_mempolicy(priv); + goto out_put_mm; + } + + /* + * After dropping the lock, there are four cases to + * consider. See the following example for explanation. + * + * +------+------+-----------+ + * | VMA1 | VMA2 | VMA3 | + * +------+------+-----------+ + * | | | | + * 4k 8k 16k 400k + * + * Suppose we drop the lock after reading VMA2 due to + * contention, then we get: + * + * last_vma_end = 16k + * + * 1) VMA2 is freed, but VMA3 exists: + * + * find_vma(mm, 16k - 1) will return VMA3. + * In this case, just continue from VMA3. + * + * 2) VMA2 still exists: + * + * find_vma(mm, 16k - 1) will return VMA2. + * Iterate the loop like the original one. + * + * 3) No more VMAs can be found: + * + * find_vma(mm, 16k - 1) will return NULL. + * No more things to do, just break. + * + * 4) (last_vma_end - 1) is the middle of a vma (VMA'): + * + * find_vma(mm, 16k - 1) will return VMA' whose range + * contains last_vma_end. + * Iterate VMA' from last_vma_end. + */ + vma = find_vma(mm, last_vma_end - 1); + /* Case 3 above */ + if (!vma) + break; + + /* Case 1 above */ + if (vma->vm_start >= last_vma_end) + continue; + + /* Case 4 above */ + if (vma->vm_end > last_vma_end) + smap_gather_stats(vma, &mss, last_vma_end); + } + /* Case 2 above */ + vma = vma->vm_next; } show_vma_header_prefix(m, priv->mm->mmap->vm_start, _
