Re: [PATCH v3] dma-buf: Split sgl by largest page-aligned chunk

[Leon Romanovsky]

On Wed, Jul 22, 2026 at 11:39:32PM +0000, dhu@xxxxxx wrote:
> From: David Hu <xuehaohu@xxxxxxxxxx>
> 
> Currently, `fill_sg_entry()` splits the scatterlist using `UINT_MAX`.
> This creates a non-page-aligned DMA length (`0xFFFFFFFF`) for the
> first entry, resulting in non-page-aligned DMA addresses for all
> subsequent entries.
> 
> While the underlying IOMMU mapping may be contiguous, hardware
> DMA engines often require explicit address alignment (e.g., page,
> cacheline, or storage sector boundaries). Passing unaligned
> addresses and lengths can cause explicit failures in DMA descriptor
> creation or silent data corruption if lower unaligned bits are
> truncated.
> 
> In addition, a non-page-aligned sgl length will trigger an edge case
> in `ib_umem_find_best_pgsz()`. In case of a discontinuity in later
> buffers, we will have a `va` with lowest bit set to 1. That will lead
> to `ib_umem_find_best_pgsz()` always return 0, and break the promise
> to find best page size for the mapping on the NIC side.
> 
> Fix this by splitting the scatterlist by the largest possible page
> aligned chunk within `UINT_MAX` (`ALIGN_DOWN(UINT_MAX, PAGE_SIZE)`).
> This ensures all scatterlist DMA addresses and lengths remain page
> aligned, while minimizing the total number of sgl entries.
> 
> Page-aligned entries allow the system to cleanly chunk payloads into
> PCIe MaxPayloadSize (MPS) (e.g., 128 bytes, 256 bytes, 512 bytes).
> As a result, this may help reduce TLP fragmentation in P2P transfers
> and alleviate potential congestion within a logical PCIe switch
> partition, especially when Relaxed Ordering is not possible due to
> hardware constraints.
> 
> Reported-by: sashiko-bot <sashiko-bot@xxxxxxxxxx>
> Closes: https://lore.kernel.org/all/20260609165431.778061F00893@xxxxxxxxxxxxxxx/
> Fixes: 3aa31a8bb11e ("dma-buf: provide phys_vec to scatter-gather mapping routine")
> Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: David Hu <xuehaohu@xxxxxxxxxx>
> ---
>  Changes in v3:
>  - Removed the type cast for `min` (David Laight)
>  - Reverted max ent size to be `ALIGN_DOWN(UINT_MAX, PAGE_SIZE)` and
>    updated commit message to reflect that (Jason Gunthorpe)
>  - Updated commit message to reflect that this also fixes an edge case
>    in `ib_umem_find_best_pgsz()`
> 
>  Changes in v2:
>  - Updated commit title and message to reflect the switch to 2G chunks
>  - Switch to using 2G as the max sg entry size as it naturally aligns
>    with most hardware boundaries, while allowing compiler optimizations
>    with bit shifts (David Laight)
>  - Optimized away division calculation for `nent`, and multiplication
>    calculation for sgl address, by dropping the `for` loop in favor of a
>    `while (length)` loop (David Laight)
>  - Dropped `min_t` in favor of `min()` to maintain a strict type
>    checking safety net (David Laight)
> 
>  drivers/dma-buf/dma-buf-mapping.c | 19 +++++++++++--------
>  1 file changed, 11 insertions(+), 8 deletions(-)

Could you please avoid sending patches as replies? It severely disrupts
the reading flow when using mutt's threaded view.

Regarding the patch,
Reviewed-by: Leon Romanovsky <leonro@xxxxxxxxxx>

Thanks