Re: [PATCH 2/2] block: allow different-pgmap pages as separate bvecs in bio_add_page

[Naman Jain <namjain@linux.microsoft.com>]

On 4/2/2026 11:00 AM, Christoph Hellwig wrote:
> On Thu, Apr 02, 2026 at 10:51:05AM +0530, Naman Jain wrote:
>> When a direct I/O request spans pages from different chunks (different
>> pgmaps), the current code rejected the second page entirely:
>>
>>      if (!zone_device_pages_have_same_pgmap(bv->bv_page, page))
>>          return 0;  // Rejection - forces bio split or I/O error
>>
>> Both chunks are regular RAM from the DMA perspective
>> (MEMORY_DEVICE_GENERIC, not P2PDMA). The only requirement is that they not
>> be merged into the same bvec segment, which patch 1/2 enforces by adding
>> the pgmap check to biovec_phys_mergeable().
>>
>> This patch allows pages from different pgmaps to be added as separate bvec
>> entries in the same bio, eliminating bio splits and I/O failures
>> when buffers span pgmap boundaries.
> 
> Which as I said we can't do in general, as different pgmaps cna have
> different DMA mapping requirements.  We might be able to relax this
> if we know multiple pgmaps can be mapped in the same way.  I.e.
> replace zone_device_pages_have_same_pgmap with
> zone_device_pages_compatible and add additional conditions to it.
> 
>> --- a/block/bio-integrity.c
>> +++ b/block/bio-integrity.c
>> @@ -231,6 +231,9 @@ int bio_integrity_add_page(struct bio *bio, struct page
>> *page,
>>          if (bip->bip_vcnt > 0) {
>>                  struct bio_vec *bv = &bip->bip_vec[bip->bip_vcnt - 1];
>>
>> +               if (is_pci_p2pdma_page(bv->bv_page) !=
>> +                   is_pci_p2pdma_page(page))
>> +                       return 0;
>>                  if (zone_device_pages_have_same_pgmap(bv->bv_page, page) &&
> 
> The above is implied by not having the same pgmap.

Thanks. If I understand correctly, here is how this would look like.
Please let me know if this is what you suggested.


diff --git a/block/bio-integrity.c b/block/bio-integrity.c
index e79eaf0477943..e54c6e06e1cbb 100644
--- a/block/bio-integrity.c
+++ b/block/bio-integrity.c
@@ -231,10 +231,10 @@ int bio_integrity_add_page(struct bio *bio, struct 
page *page,
         if (bip->bip_vcnt > 0) {
                 struct bio_vec *bv = &bip->bip_vec[bip->bip_vcnt - 1];

-               if (!zone_device_pages_have_same_pgmap(bv->bv_page, page))
+               if (!zone_device_pages_compatible(bv->bv_page, page))
                         return 0;
-
-               if (bvec_try_merge_hw_page(q, bv, page, len, offset)) {
+               if (zone_device_pages_have_same_pgmap(bv->bv_page, page) &&
+                   bvec_try_merge_hw_page(q, bv, page, len, offset)) {
                         bip->bip_iter.bi_size += len;
                         return len;
                 }
diff --git a/block/bio.c b/block/bio.c
index 77067fa346d35..0e70bb912338c 100644
--- a/block/bio.c
+++ b/block/bio.c
@@ -1034,10 +1034,10 @@ int bio_add_page(struct bio *bio, struct page *page,
         if (bio->bi_vcnt > 0) {
                 struct bio_vec *bv = &bio->bi_io_vec[bio->bi_vcnt - 1];

-               if (!zone_device_pages_have_same_pgmap(bv->bv_page, page))
+               if (!zone_device_pages_compatible(bv->bv_page, page))
                         return 0;
-
-               if (bvec_try_merge_page(bv, page, len, offset)) {
+               if (zone_device_pages_have_same_pgmap(bv->bv_page, page) &&
+                   bvec_try_merge_page(bv, page, len, offset)) {
                         bio->bi_iter.bi_size += len;
                         return len;
                 }
diff --git a/block/blk.h b/block/blk.h
index 0cb3441638284..c5710ba4c81b9 100644
--- a/block/blk.h
+++ b/block/blk.h
@@ -136,6 +136,23 @@ static inline bool biovec_phys_mergeable(struct 
request_queue *q,
         return true;
  }

+/*
+ * Check if two pages from potentially different zone device pgmaps can
+ * coexist as separate bvec entries in the same bio.
+ *
+ * The block DMA iterator (blk_dma_map_iter_start) caches the P2PDMA 
mapping
+ * state from the first segment and applies it to all subsequent 
segments, so
+ * P2PDMA and non-P2PDMA pages must never be mixed in the same bio.
+ *
+ * Other zone device types (FS_DAX, GENERIC) use the same 
dma_map_phys() path
+ * as normal RAM.  PRIVATE and COHERENT pages never appear in bios.
+ */
+static inline bool zone_device_pages_compatible(const struct page *a,
+                                               const struct page *b)
+{
+       return is_pci_p2pdma_page(a) == is_pci_p2pdma_page(b);
+}
+
  static inline bool __bvec_gap_to_prev(const struct queue_limits *lim,
                 struct bio_vec *bprv, unsigned int offset)
  {