linux-kernel - Re: [RFC PATCH] mm: filemap: avoid unnecessary major faults in filemap

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Message-ID: <ZV9wt4+F/soWxVhR@casper.infradead.org>
Date:   Thu, 23 Nov 2023 15:33:11 +0000
From:   Matthew Wilcox <willy@...radead.org>
To:     "zhangpeng (AS)" <zhangpeng362@...wei.com>
Cc:     "Huang, Ying" <ying.huang@...el.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, akpm@...ux-foundation.org,
        fengwei.yin@...el.com, aneesh.kumar@...ux.ibm.com,
        shy828301@...il.com, hughd@...gle.com, david@...hat.com,
        wangkefeng.wang@...wei.com, sunnanyong@...wei.com
Subject: Re: [RFC PATCH] mm: filemap: avoid unnecessary major faults in
 filemap_fault()

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?