On 12.06.25 18:10, Lorenzo Stoakes wrote:
On Wed, Jun 11, 2025 at 02:06:53PM +0200, David Hildenbrand wrote:
Marking PMDs that map a "normal" refcounted folios as special is
against our rules documented for vm_normal_page().
Fortunately, there are not that many pmd_special() check that can be
mislead, and most vm_normal_page_pmd()/vm_normal_folio_pmd() users that
would get this wrong right now are rather harmless: e.g., none so far
bases decisions whether to grab a folio reference on that decision.
Well, and GUP-fast will fallback to GUP-slow. All in all, so far no big
implications as it seems.
Getting this right will get more important as we use
folio_normal_page_pmd() in more places.
Fix it by teaching insert_pfn_pmd() to properly handle folios and
pfns -- moving refcount/mapcount/etc handling in there, renaming it to
insert_pmd(), and distinguishing between both cases using a new simple
"struct folio_or_pfn" structure.
Use folio_mk_pmd() to create a pmd for a folio cleanly.
Fixes: 6c88f72691f8 ("mm/huge_memory: add vmf_insert_folio_pmd()")
Signed-off-by: David Hildenbrand <da...@redhat.com>
Looks good to me, checked that the logic remains the same. Some micro
nits/thoughts below. So:
Reviewed-by: Lorenzo Stoakes <lorenzo.stoa...@oracle.com>
Thanks!
---
mm/huge_memory.c | 58 ++++++++++++++++++++++++++++++++----------------
1 file changed, 39 insertions(+), 19 deletions(-)
diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 49b98082c5401..7e3e9028873e5 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -1372,9 +1372,17 @@ vm_fault_t do_huge_pmd_anonymous_page(struct vm_fault
*vmf)
return __do_huge_pmd_anonymous_page(vmf);
}
-static int insert_pfn_pmd(struct vm_area_struct *vma, unsigned long addr,
- pmd_t *pmd, pfn_t pfn, pgprot_t prot, bool write,
- pgtable_t pgtable)
+struct folio_or_pfn {
+ union {
+ struct folio *folio;
+ pfn_t pfn;
+ };
+ bool is_folio;
+};
Interesting... I guess a memdesc world will make this easy... maybe? :)
But this is a neat way of passing this.
Another mega nit is mayyybe we could have a macro for making these like:
#define DECLARE_FOP_PFN(name_, pfn_) \
struct folio_or_pfn name_ { \
.pfn = pfn_, \
.is_folio = false, \
}
#define DECLARE_FOP_FOLIO(name_, folio_) \
struct folio_or_pfn name_ { \
.folio = folio_, \
.is_folio = true, \
}
But yeah maybe overkill for this small usage in this file.
Yeah. I suspect at some point we will convert this into a folio+idx
("page") or "pfn" approach, at which point we could also use this for
ordinary insert_pfn().
(hopefully, then we can also do pfn_t -> unsigned long)
So let's defer adding that for now.
+
+static int insert_pmd(struct vm_area_struct *vma, unsigned long addr,
+ pmd_t *pmd, struct folio_or_pfn fop, pgprot_t prot,
+ bool write, pgtable_t pgtable)
{
struct mm_struct *mm = vma->vm_mm;
pmd_t entry;
@@ -1382,8 +1390,11 @@ static int insert_pfn_pmd(struct vm_area_struct *vma,
unsigned long addr,
lockdep_assert_held(pmd_lockptr(mm, pmd));
if (!pmd_none(*pmd)) {
+ const unsigned long pfn = fop.is_folio ? folio_pfn(fop.folio) :
+ pfn_t_to_pfn(fop.pfn);
+
if (write) {
- if (pmd_pfn(*pmd) != pfn_t_to_pfn(pfn)) {
+ if (pmd_pfn(*pmd) != pfn) {
WARN_ON_ONCE(!is_huge_zero_pmd(*pmd));
return -EEXIST;
}
@@ -1396,11 +1407,19 @@ static int insert_pfn_pmd(struct vm_area_struct *vma,
unsigned long addr,
return -EEXIST;
}
- entry = pmd_mkhuge(pfn_t_pmd(pfn, prot));
- if (pfn_t_devmap(pfn))
- entry = pmd_mkdevmap(entry);
- else
- entry = pmd_mkspecial(entry);
+ if (fop.is_folio) {
+ entry = folio_mk_pmd(fop.folio, vma->vm_page_prot);
+
+ folio_get(fop.folio);
+ folio_add_file_rmap_pmd(fop.folio, &fop.folio->page, vma);
+ add_mm_counter(mm, mm_counter_file(fop.folio), HPAGE_PMD_NR);
+ } else {
+ entry = pmd_mkhuge(pfn_t_pmd(fop.pfn, prot));
Mega micro annoying nit - in above branch you have a newline after entry =, here
you don't. Maybe should add here also?
Well, it's combining all the "entry" setup in one block. But I don't
particularly care, so I'll just do it :)
--
Cheers,
David / dhildenb
