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Date:   Thu, 23 Nov 2023 15:33:11 +0000
From:   Matthew Wilcox <willy@infradead.org>
To:     "zhangpeng (AS)" <zhangpeng362@huawei.com>
Cc:     "Huang, Ying" <ying.huang@intel.com>, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org, akpm@linux-foundation.org,
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        shy828301@gmail.com, hughd@google.com, david@redhat.com,
        wangkefeng.wang@huawei.com, sunnanyong@huawei.com
Subject: Re: [RFC PATCH] mm: filemap: avoid unnecessary major faults in
 filemap_fault()
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 <87a5r4988r.fsf@yhuang6-desk2.ccr.corp.intel.com>
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On Thu, Nov 23, 2023 at 05:09:04PM +0800, zhangpeng (AS) wrote:
> > > +		pte_t *ptep = pte_offset_map_lock(vmf->vma->vm_mm, vmf->pmd,
> > > +						  vmf->address, &vmf->ptl);
> > > +		if (ptep) {
> > > +			/*
> > > +			 * Recheck pte with ptl locked as the pte can be cleared
> > > +			 * temporarily during a read/modify/write update.
> > > +			 */
> > > +			if (unlikely(!pte_none(ptep_get(ptep))))
> > > +				ret = VM_FAULT_NOPAGE;
> > > +			pte_unmap_unlock(ptep, vmf->ptl);
> > > +			if (unlikely(ret))
> > > +				return ret;
> > > +		}
> > > +
> > Need to deal with ptep == NULL.  Although that is high impossible.
> 
> If ptep == NULL, we may just need to return VM_FAULT_SIGBUS.
> I'll add it in the next version.

no?  wouldn't ptep being NULL mean that the ptep has been replaced with
a PMD entry, and thus should return NOPAGE?