net/mlx5e: SHAMPO, Fix page leak

When SHAMPO is used, a receive queue currently almost always leaks one
page on shutdown.

A page has MLX5E_SHAMPO_WQ_HEADER_PER_PAGE (8) headers. These headers
are tracked in the SHAMPO bitmap. Each page is released when the last
header index in the group is processed. During header allocation, there
can be leftovers from a page that will be used in a subsequent
allocation. This is normally fine, except for the following  scenario
(simplified a bit):

1) Allocate N new page fragments, showing only the relevant last 4
   fragments:

    0: new page
    1: new page
    2: new page
    3: new page
    4: page from previous allocation
    5: page from previous allocation
    6: page from previous allocation
    7: page from previous allocation

2) NAPI processes header indices 4-7 because they are the oldest
   allocated. Bit 7 will be set to 0.

3) Receive queue shutdown occurs. All the remaining bits are being
   iterated on to release the pages. But the page assigned to header
   indices 0-3 will not be freed due to what happened in step 2.

This patch fixes the issue by making sure that on allocation, header
fragments are always allocated in groups of
MLX5E_SHAMPO_WQ_HEADER_PER_PAGE so that there is never a partial page
left over between allocations.

A more appropriate fix would be a refactoring of
mlx5e_alloc_rx_hd_mpwqe() and mlx5e_build_shampo_hd_umr(). But this
refactoring is too big for net. It will be targeted for net-next.

Fixes: e839ac9a ("net/mlx5e: SHAMPO, Simplify header page release in teardown")
Signed-off-by: Dragos Tatulea <dtatulea@nvidia.com>
Signed-off-by: Tariq Toukan <tariqt@nvidia.com>
Link: https://patch.msgid.link/20240815071611.2211873-2-tariqt@nvidia.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>

drivers/net/ethernet/mellanox/mlx5/core/en_rx.c

@@ -735,6 +735,7 @@ static int mlx5e_alloc_rx_hd_mpwqe(struct mlx5e_rq *rq)
 	ksm_entries = bitmap_find_window(shampo->bitmap,
 					 shampo->hd_per_wqe,
 					 shampo->hd_per_wq, shampo->pi);
+	ksm_entries = ALIGN_DOWN(ksm_entries, MLX5E_SHAMPO_WQ_HEADER_PER_PAGE);
 	if (!ksm_entries)
 		return 0;