Messages in this thread | | | Subject | Re: [PATCH v3 3/3] mm: proc: smaps_rollup: do not stall write attempts on mmap_lock | From | Steven Price <> | Date | Mon, 17 Aug 2020 09:38:43 +0100 |
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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@arm.com/ > [2] https://lore.kernel.org/lkml/CANN689FtCsC71cjAjs0GPspOhgo_HRj+diWsoU1wr98YPktgWg@mail.gmail.com/ > > Signed-off-by: Chinwen Chang <chinwen.chang@mediatek.com> > CC: Steven Price <steven.price@arm.com> > CC: Michel Lespinasse <walken@google.com>
Reviewed-by: Steven Price <steven.price@arm.com>
> --- > 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
> + } > + } > + /* Case 2 above */ > + vma = vma->vm_next; > } > > show_vma_header_prefix(m, priv->mm->mmap->vm_start, >
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