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Fri, 28 Aug 2026 04:50:38 -0700 (PDT) From: Liew Rui Yan To: sashiko-bot@kernel.org Cc: aethernet65535@gmail.com, damon@lists.linux.dev, sashiko-reviews@lists.linux.dev Subject: Re: [PATCH] mm/damon: fix unconditionally skip last region Date: Fri, 28 Aug 2026 19:50:42 +0800 Message-ID: <20260828115047.332978-1-aethernet65535@gmail.com> X-Mailer: git-send-email 2.55.0 In-Reply-To: <20260828090410.40AEA1F000E9@smtp.kernel.org> References: <20260828090410.40AEA1F000E9@smtp.kernel.org> Precedence: bulk X-Mailing-List: damon@lists.linux.dev List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit On Fri, 28 Aug 2026 09:04:09 +0000 sashiko-bot@kernel.org wrote: > > diff --git a/mm/damon/core.c b/mm/damon/core.c > > index 644daf5a16560..21dc6b086c42f 100644 > > --- a/mm/damon/core.c > > +++ b/mm/damon/core.c > > @@ -2347,14 +2347,15 @@ static bool damos_skip_charged_region(struct damon_target *t, > > 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) > > + r->ar.end <= quota->charge_addr_from) { > > + if (r->ar.end == quota->charge_addr_from || > > + damon_is_last_region(r, t)) { > > + quota->charge_target_from = NULL; > > + quota->charge_addr_from = 0; > > + } > > return true; > > + } > > > > if (quota->charge_addr_from && r->ar.start < > > quota->charge_addr_from) { > > [Severity: High] > If a target's last region straddles quota->charge_addr_from (meaning > r->ar.start < quota->charge_addr_from but r->ar.end > quota->charge_addr_from), > does nesting the damon_is_last_region() check strictly inside the > r->ar.end <= quota->charge_addr_from condition cause it to bypass the reset > logic entirely? > > Because the unconditional r == damon_last_region(t) reset at the start of > the function was removed, the code now proceeds to the next block where it > attempts to skip the prefix: > > 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; > sz_to_skip = min_region_sz; > } > damon_split_region_at(t, r, sz_to_skip); > return true; > } > > If the prefix length is smaller than min_region_sz and > damon_sz_region(r) <= min_region_sz, or if the subsequent > damon_split_region_at() fails due to -ENOMEM, the function returns true early > without splitting and without clearing quota->charge_target_from. > > Does this permanently leak quota->charge_target_from? > > If the function returns true early without clearing quota->charge_target_from, > the region loop in damos_apply_target() would naturally terminate since it is > the last region, leaving the cursor permanently pointing to the current target. > > In subsequent time windows, could this leaked target repeatedly trigger the > same early return, causing all other targets to be skipped permanently > because t != quota->charge_target_from? Thank you for pointing out this issue! I just wrote a v2: ''' diff --git a/mm/damon/core.c b/mm/damon/core.c index 21dc6b086c42..02a4b2edcc4b 100644 --- a/mm/damon/core.c +++ b/mm/damon/core.c @@ -2348,14 +2348,8 @@ static bool damos_skip_charged_region(struct damon_target *t, if (t != quota->charge_target_from) return true; if (quota->charge_addr_from && - r->ar.end <= quota->charge_addr_from) { - if (r->ar.end == quota->charge_addr_from || - damon_is_last_region(r, t)) { - quota->charge_target_from = NULL; - quota->charge_addr_from = 0; - } - return true; - } + r->ar.end <= quota->charge_addr_from) + goto may_reset; if (quota->charge_addr_from && r->ar.start < quota->charge_addr_from) { @@ -2363,16 +2357,24 @@ static bool damos_skip_charged_region(struct damon_target *t, r->ar.start, min_region_sz); if (!sz_to_skip) { if (damon_sz_region(r) <= min_region_sz) - return true; + goto may_reset; sz_to_skip = min_region_sz; } - damon_split_region_at(t, r, sz_to_skip); + if (damon_split_region_at(t, r, sz_to_skip)) + goto may_reset; return true; } quota->charge_target_from = NULL; quota->charge_addr_from = 0; } return false; + +may_reset: + if (damon_is_last_region(r, t)) { + quota->charge_target_from = NULL; + quota->charge_addr_from = 0; + } + return true; } ''' > -- > Sashiko AI review ยท https://sashiko.dev/#/patchset/20260828084737.290024-1-aethernet65535@gmail.com?part=1 Best regards, Rui Yan