Re: [PATCH 03/12] mempolicy: fix migrate_pages(2) syscall return nr_failed

[Matthew Wilcox <willy@infradead.org>]

On Mon, Sep 25, 2023 at 01:24:02AM -0700, Hugh Dickins wrote:
> "man 2 migrate_pages" says "On success migrate_pages() returns the number
> of pages that could not be moved".  Although 5.3 and 5.4 commits fixed
> mbind(MPOL_MF_STRICT|MPOL_MF_MOVE*) to fail with EIO when not all pages
> could be moved (because some could not be isolated for migration),
> migrate_pages(2) was left still reporting only those pages failing at the
> migration stage, forgetting those failing at the earlier isolation stage.
> 
> Fix that by accumulating a long nr_failed count in struct queue_pages,
> returned by queue_pages_range() when it's not returning an error, for
> adding on to the nr_failed count from migrate_pages() in mm/migrate.c.
> A count of pages?  It's more a count of folios, but changing it to pages
> would entail more work (also in mm/migrate.c): does not seem justified.

I certainly see what you're saying.  If a folio is only partially mapped
(in an extreme case, the VMA is PAGE_SIZE and maps one page of a 512-page
folio), then setting nr_failed to folio_nr_pages() is misleading at best.

> +static void queue_folios_pmd(pmd_t *pmd, spinlock_t *ptl, unsigned long addr,
>  				unsigned long end, struct mm_walk *walk)
> -	__releases(ptl)
>  {
> -	int ret = 0;
>  	struct folio *folio;
>  	struct queue_pages *qp = walk->private;
> -	unsigned long flags;
>  
>  	if (unlikely(is_pmd_migration_entry(*pmd))) {
> -		ret = -EIO;
> -		goto unlock;
> +		qp->nr_failed++;
> +		return;
>  	}
>  	folio = pfn_folio(pmd_pfn(*pmd));
>  	if (is_huge_zero_page(&folio->page)) {
>  		walk->action = ACTION_CONTINUE;
> -		goto unlock;
> +		return;
>  	}
>  	if (!queue_folio_required(folio, qp))
> -		goto unlock;
> -
> -	flags = qp->flags;
> -	/* go to folio migration */
> -	if (flags & (MPOL_MF_MOVE | MPOL_MF_MOVE_ALL)) {
> -		if (!vma_migratable(walk->vma) ||
> -		    migrate_folio_add(folio, qp->pagelist, flags)) {
> -			ret = 1;
> -			goto unlock;
> -		}
> -	} else
> -		ret = -EIO;
> -unlock:
> -	spin_unlock(ptl);
> -	return ret;
> +		return;
> +	if (!(qp->flags & (MPOL_MF_MOVE | MPOL_MF_MOVE_ALL)) ||
> +	    !vma_migratable(walk->vma) ||
> +	    !migrate_folio_add(folio, qp->pagelist, qp->flags))
> +		qp->nr_failed++;

However, I think here, we would do well to increment by HPAGE_PMD_NR.
Or whatever equivalent is flavour of the week.

Bravo to the other changes.

Reviewed-by: Matthew Wilcox (Oracle) <willy@infradead.org>