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mm/damon/core: fix unconditionally skip last region

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Once quota set, the charge_{target,addr}_from unconditionally skips and
resets at the last region of the tracked target, so the last region can be
skipped even when it has not been processed.

Example:

    1. Target has 2 regions: R1 (0-100 bytes) and R2 (100-200 bytes).
    2. Quota is configured to process only 100 bytes per window.
    3. Window 1: Processes R1 (0-100).  Quota is full.  charge_{target,
       addr}_from is saved at (Target, 100).
    4. Window 2: The loop reaches R2.  Because R2 is
       damon_last_region(t), the old code unconditionally returns true,
       skipping R2 entirely and resetting the charge_{target,addr}_from.

    Result: R2 is permanently skipped even though it has never been
    processed.

However, it is important to note that this is a very minor issue.  This is
because it is triggered only when the previous window saved/kept
charge_{target,addr}_from, and in the next window, all regions except the
last region were skipped by damos_skip_charged_region().

Fix this by only resetting the charge_{target,addr}_from when last region
is reached, only skipping when it is applied or cannot split.

Link: https://lore.kernel.org/20260908134739.96919-1-sj@kernel.org
Fixes: 50585192bc ("mm/damon/schemes: skip already charged targets and regions")
Signed-off-by: Liew Rui Yan <aethernet65535@gmail.com>
Reviewed-by: SJ Park <sj@kernel.org>
Signed-off-by: SJ Park <sj@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Cc: <stable@vger.kernel.org> # v5.16.x
This commit is contained in:
Liew Rui Yan 2026-09-08 06:47:38 -07:00 committed by Andrew Morton
parent 39c0ceedd5
commit b3723b596b
1 changed files with 14 additions and 11 deletions
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25
mm/damon/core.c
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@ -2342,36 +2342,39 @@ static bool damos_skip_charged_region(struct damon_target *t,
{
struct damos_quota *quota = &s->quota;
unsigned long sz_to_skip;
bool skip = false;
/* Skip previously charged regions */
if (quota->charge_target_from) {
if (t != quota->charge_target_from)
return true;
if (r == damon_last_region(t)) {
quota->charge_target_from = NULL;
quota->charge_addr_from = 0;
return true;
}
if (quota->charge_addr_from &&
r->ar.end <= quota->charge_addr_from)
return true;
r->ar.end <= quota->charge_addr_from) {
skip = true;
goto out;
}
if (quota->charge_addr_from && r->ar.start <
quota->charge_addr_from) {
sz_to_skip = ALIGN_DOWN(quota->charge_addr_from -
r->ar.start, min_region_sz);
if (!sz_to_skip) {
if (damon_sz_region(r) <= min_region_sz)
return true;
if (damon_sz_region(r) <= min_region_sz) {
skip = true;
goto out;
}
sz_to_skip = min_region_sz;
}
damon_split_region_at(t, r, sz_to_skip);
return true;
skip = true;
}
}
out:
if (r == damon_last_region(t)) {
quota->charge_target_from = NULL;
quota->charge_addr_from = 0;
}
return false;
return skip;
}
static void damos_update_stat(struct damos *s,