Hi Paolo

On Tue, 3 Oct 2023 at 10:46, Paolo Abeni [off-list ref] wrote:

On Fri, 2023-09-22 at 17:11 +0800, Yunsheng Lin wrote:

quoted

Currently page_pool_alloc_frag() is not supported in 32-bit
arch with 64-bit DMA because of the overlap issue between
pp_frag_count and dma_addr_upper in 'struct page' for those
arches, which seems to be quite common, see [1], which means
driver may need to handle it when using fragment API.

It is assumed that the combination of the above arch with an
address space >16TB does not exist, as all those arches have
64b equivalent, it seems logical to use the 64b version for a
system with a large address space. It is also assumed that dma
address is page aligned when we are dma mapping a page aligned
buffer, see [2].

That means we're storing 12 bits of 0 at the lower end for a
dma address, we can reuse those bits for the above arches to
support 32b+12b, which is 16TB of memory.

If we make a wrong assumption, a warning is emitted so that
user can report to us.

1. https://lore.kernel.org/all/20211117075652.58299-1-linyunsheng@huawei.com/ (local)
2. https://lore.kernel.org/all/20230818145145.4b357c89@kernel.org/ (local)

Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Yunsheng Lin <redacted>
CC: Lorenzo Bianconi <lorenzo@kernel.org>
CC: Alexander Duyck <redacted>
CC: Liang Chen <redacted>
CC: Alexander Lobakin <aleksander.lobakin@intel.com>
CC: Guillaume Tucker <redacted>
CC: Matthew Wilcox <willy@infradead.org>
CC: Linux-MM <redacted>
---
 include/linux/mm_types.h        | 13 +------------
 include/net/page_pool/helpers.h | 20 ++++++++++++++------
 net/core/page_pool.c            | 14 +++++++++-----
 3 files changed, 24 insertions(+), 23 deletions(-)

diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
index 36c5b43999e6..74b49c4c7a52 100644
--- a/include/linux/mm_types.h
+++ b/include/linux/mm_types.h

@@ -125,18 +125,7 @@ struct page {
                      struct page_pool *pp;
                      unsigned long _pp_mapping_pad;
                      unsigned long dma_addr;
-                     union {
-                             /**
-                              * dma_addr_upper: might require a 64-bit
-                              * value on 32-bit architectures.
-                              */
-                             unsigned long dma_addr_upper;
-                             /**
-                              * For frag page support, not supported in
-                              * 32-bit architectures with 64-bit DMA.
-                              */
-                             atomic_long_t pp_frag_count;
-                     };
+                     atomic_long_t pp_frag_count;
              };
              struct {        /* Tail pages of compound page */
                      unsigned long compound_head;    /* Bit zero is set */

As noted by Jesper, since this is touching the super-critcal struct
page, an explicit ack from the mm people is required.

@Matthew: could you please have a look?

I think it would be nice also an explicit ack from Jesper and/or Ilias.

I am trying! Unfortunately, it's this time of the year when I have to
travel a lot.  I'll be back in 15 days from now and I will be able to
have a look

Thanks and sorry for the delay
/Ilias

Cheers,

Paolo