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SubjectRe: [PATCH 04/15] mm: remove the pgmap field from struct hmm_vma_walk
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On Wed, Aug 14, 2019 at 09:38:54AM +0200, Christoph Hellwig wrote:
> On Tue, Aug 13, 2019 at 06:36:33PM -0700, Dan Williams wrote:
> > Section alignment constraints somewhat save us here. The only example
> > I can think of a PMD not containing a uniform pgmap association for
> > each pte is the case when the pgmap overlaps normal dram, i.e. shares
> > the same 'struct memory_section' for a given span. Otherwise, distinct
> > pgmaps arrange to manage their own exclusive sections (and now
> > subsections as of v5.3). Otherwise the implementation could not
> > guarantee different mapping lifetimes.
> >
> > That said, this seems to want a better mechanism to determine "pfn is
> > ZONE_DEVICE".
>
> So I guess this patch is fine for now, and once you provide a better
> mechanism we can switch over to it?

What about the version I sent to just get rid of all the strange
put_dev_pagemaps while scanning? Odds are good we will work with only
a single pagemap, so it makes some sense to cache it once we find it?

diff --git a/mm/hmm.c b/mm/hmm.c
index 9a908902e4cc38..4e30128c23a505 100644
--- a/mm/hmm.c
+++ b/mm/hmm.c
@@ -497,10 +497,6 @@ static int hmm_vma_handle_pmd(struct mm_walk *walk,
}
pfns[i] = hmm_device_entry_from_pfn(range, pfn) | cpu_flags;
}
- if (hmm_vma_walk->pgmap) {
- put_dev_pagemap(hmm_vma_walk->pgmap);
- hmm_vma_walk->pgmap = NULL;
- }
hmm_vma_walk->last = end;
return 0;
#else
@@ -604,10 +600,6 @@ static int hmm_vma_handle_pte(struct mm_walk *walk, unsigned long addr,
return 0;

fault:
- if (hmm_vma_walk->pgmap) {
- put_dev_pagemap(hmm_vma_walk->pgmap);
- hmm_vma_walk->pgmap = NULL;
- }
pte_unmap(ptep);
/* Fault any virtual address we were asked to fault */
return hmm_vma_walk_hole_(addr, end, fault, write_fault, walk);
@@ -690,16 +682,6 @@ static int hmm_vma_walk_pmd(pmd_t *pmdp,
return r;
}
}
- if (hmm_vma_walk->pgmap) {
- /*
- * We do put_dev_pagemap() here and not in hmm_vma_handle_pte()
- * so that we can leverage get_dev_pagemap() optimization which
- * will not re-take a reference on a pgmap if we already have
- * one.
- */
- put_dev_pagemap(hmm_vma_walk->pgmap);
- hmm_vma_walk->pgmap = NULL;
- }
pte_unmap(ptep - 1);

hmm_vma_walk->last = addr;
@@ -751,10 +733,6 @@ static int hmm_vma_walk_pud(pud_t *pudp,
pfns[i] = hmm_device_entry_from_pfn(range, pfn) |
cpu_flags;
}
- if (hmm_vma_walk->pgmap) {
- put_dev_pagemap(hmm_vma_walk->pgmap);
- hmm_vma_walk->pgmap = NULL;
- }
hmm_vma_walk->last = end;
return 0;
}
@@ -1026,6 +1004,14 @@ long hmm_range_fault(struct hmm_range *range, unsigned int flags)
/* Keep trying while the range is valid. */
} while (ret == -EBUSY && range->valid);

+ /*
+ * We do put_dev_pagemap() here so that we can leverage
+ * get_dev_pagemap() optimization which will not re-take a
+ * reference on a pgmap if we already have one.
+ */
+ if (hmm_vma_walk->pgmap)
+ put_dev_pagemap(hmm_vma_walk->pgmap);
+
if (ret) {
unsigned long i;

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