On Mon, 2020-08-17 at 09:38 +0100, Steven Price wrote: > On 15/08/2020 07:20, Chinwen Chang wrote: > > 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. > > > > Change since v1: > > - If current VMA is freed after dropping the lock, it will return > > - incomplete result. To fix this issue, refine the code flow as > > - suggested by Steve. [1] > > > > Change since v2: > > - When getting back the mmap lock, the address where you stopped last > > - time could now be in the middle of a vma. Add one more check to handle > > - this case as suggested by Michel. [2] > > > > [1] https://lore.kernel.org/lkml/bf40676e-b14b-44cd-75ce-419c70194783@xxxxxxx/ > > [2] https://lore.kernel.org/lkml/CANN689FtCsC71cjAjs0GPspOhgo_HRj+diWsoU1wr98YPktgWg@xxxxxxxxxxxxxx/ > > > > Signed-off-by: Chinwen Chang <chinwen.chang@xxxxxxxxxxxx> > > CC: Steven Price <steven.price@xxxxxxx> > > CC: Michel Lespinasse <walken@xxxxxxxxxx> > > Reviewed-by: Steven Price <steven.price@xxxxxxx> > > > --- > > fs/proc/task_mmu.c | 73 +++++++++++++++++++++++++++++++++++++++++++++++++++--- > > 1 file changed, 70 insertions(+), 3 deletions(-) > > > > diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c > > index 76e623a..945904e 100644 > > --- a/fs/proc/task_mmu.c > > +++ b/fs/proc/task_mmu.c > > @@ -846,7 +846,7 @@ static int show_smaps_rollup(struct seq_file *m, void *v) > > struct mem_size_stats mss; > > struct mm_struct *mm; > > struct vm_area_struct *vma; > > - unsigned long last_vma_end = 0; > > + unsigned long last_vma_end = 0, last_stopped = 0; > > int ret = 0; > > > > priv->task = get_proc_task(priv->inode); > > @@ -867,9 +867,76 @@ static int show_smaps_rollup(struct seq_file *m, void *v) > > > > hold_task_mempolicy(priv); > > > > - for (vma = priv->mm->mmap; vma; vma = vma->vm_next) { > > - smap_gather_stats(vma, &mss, 0); > > + for (vma = priv->mm->mmap; vma;) { > > + smap_gather_stats(vma, &mss, last_stopped); > > + last_stopped = 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) { > > + last_stopped = last_vma_end; > > + continue; > > Note that instead of having last_stopped, you could replace the above > with a direct call: > > smap_gather_stats(vma, &mss, last_vma_end); > > I'm not sure which is cleaner though. last_stopped is a bit messy (it's > easily confused with last_vma_end), but having just the one call site > for smap_gather_stats() is nice too. > > Steve > Hi Steve, I think your idea is better. Let me try refactoring for further reviews. Thanks for your kind suggestion:) Chinwen > > + } > > + } > > + /* Case 2 above */ > > + vma = vma->vm_next; > > } > > > > show_vma_header_prefix(m, priv->mm->mmap->vm_start, > > >
