Re: [PATCH v3 1/5] mm: pagewalk: add the ability to install PTEs

[Vlastimil Babka <vbabka@suse.cz>]

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@google.com>
> Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>

Reviewed-by: Vlastimil Babka <vbabka@suse.cz>

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. */
}

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

>  		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.

Thanks!