Re: [PATCH v12 5/6] khugepaged: enable collapse pmd for pte-mapped THP

From: Oleg Nesterov <oleg@redhat.com>
To: Song Liu <songliubraving@fb.com>
Cc: Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	Linux MM <linux-mm@kvack.org>,
	Andrew Morton <akpm@linux-foundation.org>,
	Matthew Wilcox <matthew.wilcox@oracle.com>,
	"Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
	Kernel Team <Kernel-team@fb.com>,
	William Kucharski <william.kucharski@oracle.com>,
	"srikar@linux.vnet.ibm.com" <srikar@linux.vnet.ibm.com>
Subject: Re: [PATCH v12 5/6] khugepaged: enable collapse pmd for pte-mapped THP
Date: Fri, 9 Aug 2019 17:24:04 +0200	[thread overview]
Message-ID: <20190809152404.GA21489@redhat.com> (raw)
In-Reply-To: <770B3C29-CE8F-4228-8992-3C6E2B5487B6@fb.com>

On 08/08, Song Liu wrote:
>
> > On Aug 8, 2019, at 9:33 AM, Oleg Nesterov <oleg@redhat.com> wrote:
> >
> >> +	for (i = 0, addr = haddr; i < HPAGE_PMD_NR; i++, addr += PAGE_SIZE) {
> >> +		pte_t *pte = pte_offset_map(pmd, addr);
> >> +		struct page *page;
> >> +
> >> +		if (pte_none(*pte))
> >> +			continue;
> >> +
> >> +		page = vm_normal_page(vma, addr, *pte);

just noticed... shouldn't you also check pte_present() before
vm_normal_page() ?

> >> +		if (!page || !PageCompound(page))
> >> +			return;
> >> +
> >> +		if (!hpage) {
> >> +			hpage = compound_head(page);
> >
> > OK,
> >
> >> +			if (hpage->mapping != vma->vm_file->f_mapping)
> >> +				return;
> >
> > is it really possible? May be WARN_ON(hpage->mapping != vm_file->f_mapping)
> > makes more sense ?
>
> I haven't found code paths lead to this,

Neither me, that is why I asked. I think this should not be possible,
but again this is not my area.

> but this is technically possible.
> This pmd could contain subpages from different THPs.

Then please explain how this can happen ?

> The __replace_page()
> function in uprobes.c creates similar pmd.

No it doesn't,

> Current uprobe code won't really create this problem, because
> !PageCompound() check above is sufficient. But it won't be difficult to
> modify uprobe code to break this.

I bet it will be a) difficult and b) the very idea to do this would be wrong.

> For this code to be accurate and safe,
> I think both this check and the one below are necessary.

I didn't suggest to remove these checks.

> Also, this code
> is not on any critical path, so the overhead should be negligible.

I do not care about overhead. But I do care about a poor reader like me
who will try to understand this code.

If you too do not understand how a THP page can have a different mapping
then use VM_WARN or at least add a comment to explain that this is not
supposed to happen!

> Does this make sense?

Not to me :/

Oleg.