Re: [PATCH v2 2/9] nvme: set max_hw_wzeroes_unmap_sectors if device supports DEAC bit

[Robert Pang <robertpang@google.com>]

Dear Zhang Yi,

In reviewing your patch series implementing support for the
FALLOC_FL_WRITE_ZEROES flag, I noted the logic propagating
max_write_zeroes_sectors to max_hw_wzeroes_unmap_sectors in commit 545fb46e5bc6
"nvme: set max_hw_wzeroes_unmap_sectors if device supports DEAC bit" [1]. This
appears to be intended for devices that support the Write Zeroes command
alongside the DEAC bit to indicate unmap capability.

Furthermore, within core.c, the NVME_QUIRK_DEALLOCATE_ZEROES quirk already
identifies devices that deterministically return zeroes after a deallocate
command [2]. This quirk currently enables Write Zeroes support via discard in
existing implementations [3, 4].

Given this, would it be appropriate to respect NVME_QUIRK_DEALLOCATE_ZEROES also
to enable unmap Write Zeroes for these devices, following the prior commit
6e02318eaea5 "nvme: add support for the Write Zeroes command" [5]? I have
included a proposed change to nvme_update_ns_info_block() below for your
consideration.

Best regards
Robert Pang

diff --git a/drivers/nvme/host/core.c b/drivers/nvme/host/core.c
index f5ebcaa2f859..9c7e2cabfab3 100644
--- a/drivers/nvme/host/core.c
+++ b/drivers/nvme/host/core.c
@@ -2422,7 +2422,9 @@ static int nvme_update_ns_info_block(struct nvme_ns *ns,
         * require that, it must be a no-op if reads from deallocated data
         * do not return zeroes.
         */
-       if ((id->dlfeat & 0x7) == 0x1 && (id->dlfeat & (1 << 3))) {
+       if ((id->dlfeat & 0x7) == 0x1 && (id->dlfeat & (1 << 3)) ||
+           (ns->ctrl->quirks & NVME_QUIRK_DEALLOCATE_ZEROES) &&
+           (ns->ctrl->oncs & NVME_CTRL_ONCS_DSM)) {
                ns->head->features |= NVME_NS_DEAC;
                lim.max_hw_wzeroes_unmap_sectors = lim.max_write_zeroes_sectors;
        }

[1] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=545fb46e5bc6
[2] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/drivers/nvme/host/nvme.h#n72
[3] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/drivers/nvme/host/core.c#n938
[4] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/drivers/nvme/host/core.c#n2122
[5] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=6e02318eaea5