Peng Zhang <zhangpeng362@xxxxxxxxxx> writes: > From: ZhangPeng <zhangpeng362@xxxxxxxxxx> > > The major fault occurred when using mlockall(MCL_CURRENT | MCL_FUTURE) > in application, which leading to an unexpected issue[1]. > > This caused by temporarily cleared PTE during a read+clear/modify/write > update of the PTE, eg, do_numa_page()/change_pte_range(). > > For the data segment of the user-mode program, the global variable area > is a private mapping. After the pagecache is loaded, the private anonymous > page is generated after the COW is triggered. Mlockall can lock COW pages > (anonymous pages), but the original file pages cannot be locked and may > be reclaimed. If the global variable (private anon page) is accessed when > vmf->pte is zeroed in numa fault, a file page fault will be triggered. > At this time, the original private file page may have been reclaimed. > If the page cache is not available at this time, a major fault will be > triggered and the file will be read, causing additional overhead. > > This issue affects our traffic analysis service. The inbound traffic is > heavy. If a major fault occurs, the I/O schedule is triggered and the > original I/O is suspended. Generally, the I/O schedule is 0.7 ms. If > other applications are operating disks, the system needs to wait for > more than 10 ms. However, the inbound traffic is heavy and the NIC buffer > is small. As a result, packet loss occurs. But the traffic analysis service > can't tolerate packet loss. > > Fix this by holding PTL and rechecking the PTE in filemap_fault() before > triggering a major fault. We do this check only if vma is VM_LOCKED to > reduce the performance impact in common scenarios. > > In our product environment, there were 7 major faults every 12 hours. > After the patch is applied, no major fault have been triggered. > > Testing file page read and write page fault performance in ext4 and > ramdisk using will-it-scale[2] on a x86 physical machine. The data is > the average change compared with the mainline after the patch is applied. > The test results are within the range of fluctuation. We do this check > only if vma is VM_LOCKED, therefore, no performance regressions is caused > for most common cases. > > The test results are as follows: > processes processes_idle threads threads_idle > ext4 private file write: 0.22% 0.26% 1.21% -0.15% > ext4 private file read: 0.03% 1.00% 1.39% 0.34% > ext4 shared file write: -0.50% -0.02% -0.14% -0.02% > ramdisk private file write: 0.07% 0.02% 0.53% 0.04% > ramdisk private file read: 0.01% 1.60% -0.32% -0.02% > > [1] https://lore.kernel.org/linux-mm/9e62fd9a-bee0-52bf-50a7-498fa17434ee@xxxxxxxxxx/ > [2] https://github.com/antonblanchard/will-it-scale/ > > Suggested-by: "Huang, Ying" <ying.huang@xxxxxxxxx> > Suggested-by: David Hildenbrand <david@xxxxxxxxxx> > Signed-off-by: ZhangPeng <zhangpeng362@xxxxxxxxxx> > Signed-off-by: Kefeng Wang <wangkefeng.wang@xxxxxxxxxx> LGTM, Thanks! Feel free to add Reviewed-by: "Huang, Ying" <ying.huang@xxxxxxxxx> > --- > v3->v4: > - Update the performance data and commit message > - Check PTE without lock firstly per Huang, Ying > - Update comments for recheck function per David Hildenbrand > - Simply return 0 to make it easier to read per David Hildenbrand > - Check !FAULT_FLAG_ORIG_PTE_VALID instead of pmd_none() > > v2->v3: > - Do this check only if vma is VM_LOCKED per David Hildenbrand > - Hold PTL and recheck the PTE > - Place the recheck code in a new function filemap_fault_recheck_pte() > > v1->v2: > - Add more test results per Huang, Ying > - Add more comments before check PTE per Huang, Ying, David Hildenbrand > and Yin Fengwei > - Change pte_offset_map_nolock to pte_offset_map as the PTL won't > be used > > RFC->v1: > - Add error handling when ptep == NULL per Huang, Ying and Matthew > Wilcox > - Check the PTE without acquiring PTL in filemap_fault(), suggested by > Huang, Ying and Yin Fengwei > - Add pmd_none() check before PTE map > - Update commit message and add performance test information > > mm/filemap.c | 46 ++++++++++++++++++++++++++++++++++++++++++++++ > 1 file changed, 46 insertions(+) > > diff --git a/mm/filemap.c b/mm/filemap.c > index b4858d89f1b1..31ab455c4537 100644 > --- a/mm/filemap.c > +++ b/mm/filemap.c > @@ -3181,6 +3181,48 @@ static struct file *do_async_mmap_readahead(struct vm_fault *vmf, > return fpin; > } > > +static vm_fault_t filemap_fault_recheck_pte_none(struct vm_fault *vmf) > +{ > + struct vm_area_struct *vma = vmf->vma; > + vm_fault_t ret = 0; > + pte_t *ptep; > + > + /* > + * We might have COW'ed a pagecache folio and might now have an mlocked > + * anon folio mapped. The original pagecache folio is not mlocked and > + * might have been evicted. During a read+clear/modify/write update of > + * the PTE, such as done in do_numa_page()/change_pte_range(), we > + * temporarily clear the PTE under PT lock and might detect it here as > + * "none" when not holding the PT lock. > + * > + * Not rechecking the PTE under PT lock could result in an unexpected > + * major fault in an mlock'ed region. Recheck only for this special > + * scenario while holding the PT lock, to not degrade non-mlocked > + * scenarios. Recheck the PTE without PT lock firstly, thereby reducing > + * the number of times we hold PT lock. > + */ > + if (!(vma->vm_flags & VM_LOCKED)) > + return 0; > + > + if (!(vmf->flags & FAULT_FLAG_ORIG_PTE_VALID)) > + return 0; > + > + ptep = pte_offset_map(vmf->pmd, vmf->address); > + if (unlikely(!ptep)) > + return VM_FAULT_NOPAGE; > + > + if (unlikely(!pte_none(ptep_get_lockless(ptep)))) { > + ret = VM_FAULT_NOPAGE; > + } else { > + spin_lock(vmf->ptl); > + if (unlikely(!pte_none(ptep_get(ptep)))) > + ret = VM_FAULT_NOPAGE; > + spin_unlock(vmf->ptl); > + } > + pte_unmap(ptep); > + return ret; > +} > + > /** > * filemap_fault - read in file data for page fault handling > * @vmf: struct vm_fault containing details of the fault > @@ -3236,6 +3278,10 @@ vm_fault_t filemap_fault(struct vm_fault *vmf) > mapping_locked = true; > } > } else { > + ret = filemap_fault_recheck_pte_none(vmf); > + if (unlikely(ret)) > + return ret; > + > /* No page in the page cache at all */ > count_vm_event(PGMAJFAULT); > count_memcg_event_mm(vmf->vma->vm_mm, PGMAJFAULT);
