Re: [PATCH v4] filemap: avoid unnecessary major faults in filemap_fault()

[David Hildenbrand <david@redhat.com>]

On 06.03.24 09:38, Peng Zhang wrote:
> From: ZhangPeng <zhangpeng362@huawei.com>
> 
> 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@huawei.com/
> [2] https://github.com/antonblanchard/will-it-scale/
> 
> Suggested-by: "Huang, Ying" <ying.huang@intel.com>
> Suggested-by: David Hildenbrand <david@redhat.com>
> Signed-off-by: ZhangPeng <zhangpeng362@huawei.com>
> Signed-off-by: Kefeng Wang <wangkefeng.wang@huawei.com>
> ---
> 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);

I assume the assumption is that vmf->ptl is still set from 
handle_pte_fault(). But in the meantime, we did a pte_unmap(vmf->pte).

Looking at other users of vmf->ptl, this here sticks out.

I would suggest replacing the pte_offset_map() above by a 
pte_offset_map_nolock() that gives us the ptl.


With that

Reviewed-by: David Hildenbrand <david@redhat.com>

-- 
Cheers,

David / dhildenb