linux-kernel - Re: [PATCH v2 6/7] mm: Batch around can_change_pte

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Message-ID: <7cf8235e-21f7-4643-82c4-82ad57d99b98@lucifer.local>
Date: Tue, 29 Apr 2025 14:57:19 +0100
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: David Hildenbrand <david@...hat.com>
Cc: Dev Jain <dev.jain@....com>, akpm@...ux-foundation.org,
        ryan.roberts@....com, willy@...radead.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, catalin.marinas@....com, will@...nel.org,
        Liam.Howlett@...cle.com, vbabka@...e.cz, jannh@...gle.com,
        anshuman.khandual@....com, peterx@...hat.com, joey.gouly@....com,
        ioworker0@...il.com, baohua@...nel.org, kevin.brodsky@....com,
        quic_zhenhuah@...cinc.com, christophe.leroy@...roup.eu,
        yangyicong@...ilicon.com, linux-arm-kernel@...ts.infradead.org,
        hughd@...gle.com, yang@...amperecomputing.com, ziy@...dia.com
Subject: Re: [PATCH v2 6/7] mm: Batch around can_change_pte_writable()

On Tue, Apr 29, 2025 at 11:27:43AM +0200, David Hildenbrand wrote:
> On 29.04.25 11:19, David Hildenbrand wrote:
> >
> > >    #include "internal.h"
> > > -bool can_change_pte_writable(struct vm_area_struct *vma, unsigned long addr,
> > > -			     pte_t pte)
> > > +bool can_change_ptes_writable(struct vm_area_struct *vma, unsigned long addr,
> > > +			      pte_t pte, struct folio *folio, unsigned int nr)
> > >    {
> > >    	struct page *page;
> > > @@ -67,8 +67,9 @@ bool can_change_pte_writable(struct vm_area_struct *vma, unsigned long addr,
> > >    		 * write-fault handler similarly would map them writable without
> > >    		 * any additional checks while holding the PT lock.
> > >    		 */
> > > -		page = vm_normal_page(vma, addr, pte);
> > > -		return page && PageAnon(page) && PageAnonExclusive(page);
> > > +		if (!folio)
> > > +			folio = vm_normal_folio(vma, addr, pte);
> > > +		return folio_test_anon(folio) && !folio_maybe_mapped_shared(folio);
> >
> > Oh no, now I spot it. That is horribly wrong.
> >
> > Please understand first what you are doing.
>
> Also, would expect that the cow.c selftest would catch that:
>
> "vmsplice() + unmap in child with mprotect() optimization"
>
> After fork() we have a R/O PTE in the parent. Our child then uses vmsplice()
> and unmaps the R/O PTE, meaning it is only left mapped by the parent.
>
> ret = mprotect(mem, size, PROT_READ);
> ret |= mprotect(mem, size, PROT_READ|PROT_WRITE);
>
> should turn the PTE writable, although it shouldn't.

This makes me concerned about the stability of this series as a whole...

>
> If that test case does not detect the issue you're introducing, we should
> look into adding a test case that detects it.

There are 25 tests that fail for the cow self-test with this series
applied:

# [RUN] vmsplice() + unmap in child with mprotect() optimization ... with base page
# [RUN] vmsplice() + unmap in child with mprotect() optimization ... with PTE-mapped THP (16 kB)
# [RUN] vmsplice() + unmap in child with mprotect() optimization ... with single PTE of THP (16 kB)
# [RUN] vmsplice() + unmap in child with mprotect() optimization ... with partially shared THP (16 kB)
# [RUN] vmsplice() + unmap in child with mprotect() optimization ... with PTE-mapped THP (32 kB)
# [RUN] vmsplice() + unmap in child with mprotect() optimization ... with single PTE of THP (32 kB)
# [RUN] vmsplice() + unmap in child with mprotect() optimization ... with partially shared THP (32 kB)
# [RUN] vmsplice() + unmap in child with mprotect() optimization ... with PTE-mapped THP (64 kB)
# [RUN] vmsplice() + unmap in child with mprotect() optimization ... with single PTE of THP (64 kB)
# [RUN] vmsplice() + unmap in child with mprotect() optimization ... with partially shared THP (64 kB)
# [RUN] vmsplice() + unmap in child with mprotect() optimization ... with PTE-mapped THP (128 kB)
# [RUN] vmsplice() + unmap in child with mprotect() optimization ... with single PTE of THP (128 kB)
# [RUN] vmsplice() + unmap in child with mprotect() optimization ... with partially shared THP (128 kB)
# [RUN] vmsplice() + unmap in child with mprotect() optimization ... with PTE-mapped THP (256 kB)
# [RUN] vmsplice() + unmap in child with mprotect() optimization ... with single PTE of THP (256 kB)
# [RUN] vmsplice() + unmap in child with mprotect() optimization ... with partially shared THP (256 kB)
# [RUN] vmsplice() + unmap in child with mprotect() optimization ... with PTE-mapped THP (512 kB)
# [RUN] vmsplice() + unmap in child with mprotect() optimization ... with single PTE of THP (512 kB)
# [RUN] vmsplice() + unmap in child with mprotect() optimization ... with partially shared THP (512 kB)
# [RUN] vmsplice() + unmap in child with mprotect() optimization ... with PTE-mapped THP (1024 kB)
# [RUN] vmsplice() + unmap in child with mprotect() optimization ... with single PTE of THP (1024 kB)
# [RUN] vmsplice() + unmap in child with mprotect() optimization ... with partially shared THP (1024 kB)
# [RUN] vmsplice() + unmap in child with mprotect() optimization ... with PTE-mapped THP (2048 kB)
# [RUN] vmsplice() + unmap in child with mprotect() optimization ... with single PTE of THP (2048 kB)
# [RUN] vmsplice() + unmap in child with mprotect() optimization ... with partially shared THP (2048 kB)


Dev, please take a little more time to test your series :) the current
patch set doesn't compile and needs fixes applied to do so, and we're at
v2, and you've clearly not run self-tests as these also fail.

Please ensure you do a smoke test and check compilation before sending out,
as well as running self tests also.

Thanks, Lorenzo

>
> --
> Cheers,
>
> David / dhildenb
>