Message-ID: <046fe24f-6768-4b57-80bd-028514b0db51@lucifer.local>
Date: Fri, 25 Oct 2024 22:58:28 +0100
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: Vlastimil Babka <vbabka@...e.cz>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
        Suren Baghdasaryan <surenb@...gle.com>,
        "Liam R . Howlett" <Liam.Howlett@...cle.com>,
        Matthew Wilcox <willy@...radead.org>,
        "Paul E . McKenney" <paulmck@...nel.org>, Jann Horn <jannh@...gle.com>,
        David Hildenbrand <david@...hat.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, Muchun Song <muchun.song@...ux.dev>,
        Richard Henderson <richard.henderson@...aro.org>,
        Matt Turner <mattst88@...il.com>,
        Thomas Bogendoerfer <tsbogend@...ha.franken.de>,
        "James E . J . Bottomley" <James.Bottomley@...senpartnership.com>,
        Helge Deller <deller@....de>, Chris Zankel <chris@...kel.net>,
        Max Filippov <jcmvbkbc@...il.com>, Arnd Bergmann <arnd@...nel.org>,
        linux-alpha@...r.kernel.org, linux-mips@...r.kernel.org,
        linux-parisc@...r.kernel.org, linux-arch@...r.kernel.org,
        Shuah Khan <shuah@...nel.org>, Christian Brauner <brauner@...nel.org>,
        linux-kselftest@...r.kernel.org,
        Sidhartha Kumar <sidhartha.kumar@...cle.com>,
        Jeff Xu <jeffxu@...omium.org>, Christoph Hellwig <hch@...radead.org>,
        linux-api@...r.kernel.org, John Hubbard <jhubbard@...dia.com>
Subject: Re: [PATCH v3 1/5] mm: pagewalk: add the ability to install PTEs

On Fri, Oct 25, 2024 at 08:13:26PM +0200, Vlastimil Babka wrote:
> On 10/23/24 18:24, Lorenzo Stoakes wrote:
> > The existing generic pagewalk logic permits the walking of page tables,
> > invoking callbacks at individual page table levels via user-provided
> > mm_walk_ops callbacks.
> >
> > This is useful for traversing existing page table entries, but precludes
> > the ability to establish new ones.
> >
> > Existing mechanism for performing a walk which also installs page table
> > entries if necessary are heavily duplicated throughout the kernel, each
> > with semantic differences from one another and largely unavailable for use
> > elsewhere.
> >
> > Rather than add yet another implementation, we extend the generic pagewalk
> > logic to enable the installation of page table entries by adding a new
> > install_pte() callback in mm_walk_ops. If this is specified, then upon
> > encountering a missing page table entry, we allocate and install a new one
> > and continue the traversal.
> >
> > If a THP huge page is encountered at either the PMD or PUD level we split
> > it only if there are ops->pte_entry() (or ops->pmd_entry at PUD level),
> > otherwise if there is only an ops->install_pte(), we avoid the unnecessary
> > split.
> >
> > We do not support hugetlb at this stage.
> >
> > If this function returns an error, or an allocation fails during the
> > operation, we abort the operation altogether. It is up to the caller to
> > deal appropriately with partially populated page table ranges.
> >
> > If install_pte() is defined, the semantics of pte_entry() change - this
> > callback is then only invoked if the entry already exists. This is a useful
> > property, as it allows a caller to handle existing PTEs while installing
> > new ones where necessary in the specified range.
> >
> > If install_pte() is not defined, then there is no functional difference to
> > this patch, so all existing logic will work precisely as it did before.
> >
> > As we only permit the installation of PTEs where a mapping does not already
> > exist there is no need for TLB management, however we do invoke
> > update_mmu_cache() for architectures which require manual maintenance of
> > mappings for other CPUs.
> >
> > We explicitly do not allow the existing page walk API to expose this
> > feature as it is dangerous and intended for internal mm use only. Therefore
> > we provide a new walk_page_range_mm() function exposed only to
> > mm/internal.h.
> >
> > Reviewed-by: Jann Horn <jannh@...gle.com>
> > Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
>
> Reviewed-by: Vlastimil Babka <vbabka@...e.cz>

Thanks!

>
> Just a small subjective suggestion in case you agree and there's a respin or
> followups:
>
> > @@ -109,18 +131,19 @@ static int walk_pmd_range(pud_t *pud, unsigned long addr, unsigned long end,
> >
> >  		if (walk->action == ACTION_AGAIN)
> >  			goto again;
> > -
> > -		/*
> > -		 * Check this here so we only break down trans_huge
> > -		 * pages when we _need_ to
> > -		 */
> > -		if ((!walk->vma && (pmd_leaf(*pmd) || !pmd_present(*pmd))) ||
> > -		    walk->action == ACTION_CONTINUE ||
> > -		    !(ops->pte_entry))
> > +		if (walk->action == ACTION_CONTINUE)
> >  			continue;
> > +		if (!ops->install_pte && !ops->pte_entry)
> > +			continue; /* Nothing to do. */
> > +		if (!ops->pte_entry && ops->install_pte &&
> > +		    pmd_present(*pmd) &&
> > +		    (pmd_trans_huge(*pmd) || pmd_devmap(*pmd)))
> > +			continue; /* Avoid unnecessary split. */
>
> Much better now, thanks, but maybe the last 2 parts could be:
>
> if (!ops->pte_entry) {
> 	if (!ops->install_pte)
> 		continue; /* Nothing to do. */
> 	else if (pmd_present(*pmd)
> 		 && (pmd_trans_huge(*pmd) || pmd_devmap(*pmd)))
> 		continue; /* Avoid unnecessary split. */
> }

I quite liked separating out the 'nothing to do' vs. the 'unnecessary split'
cases, but I agree it makes it harder to see that the 2nd case is an 'install
pte ONLY' case.

Yeah so I think your version is better, but maybe we can find a way to be more
expressive somehow... if we could declare vars mid-way thhrough it'd be easier
:P

Will improve on respin if it comes up

>
> Or at least put !ops->pte_entry first in both conditions?

Ack yeah that'd be better!

>
> >  		if (walk->vma)
> >  			split_huge_pmd(walk->vma, pmd, addr);
> > +		else if (pmd_leaf(*pmd) || !pmd_present(*pmd))
> > +			continue; /* Nothing to do. */
> >
> >  		err = walk_pte_range(pmd, addr, next, walk);
> >  		if (err)
> > @@ -148,11 +171,14 @@ static int walk_pud_range(p4d_t *p4d, unsigned long addr, unsigned long end,
> >   again:
> >  		next = pud_addr_end(addr, end);
> >  		if (pud_none(*pud)) {
> > -			if (ops->pte_hole)
> > +			if (ops->install_pte)
> > +				err = __pmd_alloc(walk->mm, pud, addr);
> > +			else if (ops->pte_hole)
> >  				err = ops->pte_hole(addr, next, depth, walk);
> >  			if (err)
> >  				break;
> > -			continue;
> > +			if (!ops->install_pte)
> > +				continue;
> >  		}
> >
> >  		walk->action = ACTION_SUBTREE;
> > @@ -164,14 +190,20 @@ static int walk_pud_range(p4d_t *p4d, unsigned long addr, unsigned long end,
> >
> >  		if (walk->action == ACTION_AGAIN)
> >  			goto again;
> > -
> > -		if ((!walk->vma && (pud_leaf(*pud) || !pud_present(*pud))) ||
> > -		    walk->action == ACTION_CONTINUE ||
> > -		    !(ops->pmd_entry || ops->pte_entry))
> > +		if (walk->action == ACTION_CONTINUE)
> >  			continue;
> > +		if (!ops->install_pte && !ops->pte_entry && !ops->pmd_entry)
> > +			continue;  /* Nothing to do. */
> > +		if (!ops->pmd_entry && !ops->pte_entry && ops->install_pte &&
> > +		    pud_present(*pud) &&
> > +		    (pud_trans_huge(*pud) || pud_devmap(*pud)))
> > +			continue; /* Avoid unnecessary split. */
>
> Ditto.

Ack!

>
> Thanks!
>

Cheers!

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