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Message-ID: <CAHS8izMYaHf-3zeVcM_73_KSMCpA5vds-NtRjNt0d8VsMfczQw@mail.gmail.com>
Date: Mon, 4 Nov 2019 13:19:18 -0800
From: Mina Almasry <almasrymina@...gle.com>
To: Mike Kravetz <mike.kravetz@...cle.com>
Cc: shuah <shuah@...nel.org>, open list <linux-kernel@...r.kernel.org>,
linux-mm@...ck.org, linux-kselftest@...r.kernel.org,
cgroups@...r.kernel.org,
Aneesh Kumar <aneesh.kumar@...ux.vnet.ibm.com>
Subject: Re: [PATCH v8 5/9] hugetlb: disable region_add file_region coalescing
On Mon, Nov 4, 2019 at 1:15 PM Mike Kravetz <mike.kravetz@...cle.com> wrote:
>
> On 11/4/19 1:04 PM, Mina Almasry wrote:
> > On Fri, Nov 1, 2019 at 4:23 PM Mike Kravetz <mike.kravetz@...cle.com> wrote:
> >>
> >> On 10/29/19 6:36 PM, Mina Almasry wrote:
> >>> static long add_reservation_in_range(struct resv_map *resv, long f, long t,
> >>> - bool count_only)
> >>> + long *regions_needed, bool count_only)
> >>> {
> >>> - long chg = 0;
> >>> + long add = 0;
> >>> struct list_head *head = &resv->regions;
> >>> + long last_accounted_offset = f;
> >>> struct file_region *rg = NULL, *trg = NULL, *nrg = NULL;
> >>>
> >>> - /* Locate the region we are before or in. */
> >>> - list_for_each_entry (rg, head, link)
> >>> - if (f <= rg->to)
> >>> - break;
> >>> + if (regions_needed)
> >>> + *regions_needed = 0;
> >>>
> >>> - /* Round our left edge to the current segment if it encloses us. */
> >>> - if (f > rg->from)
> >>> - f = rg->from;
> >>> -
> >>> - chg = t - f;
> >>> + /* In this loop, we essentially handle an entry for the range
> >>> + * [last_accounted_offset, rg->from), at every iteration, with some
> >>> + * bounds checking.
> >>> + */
> >>> + list_for_each_entry_safe(rg, trg, head, link) {
> >>> + /* Skip irrelevant regions that start before our range. */
> >>> + if (rg->from < f) {
> >>> + /* If this region ends after the last accounted offset,
> >>> + * then we need to update last_accounted_offset.
> >>> + */
> >>> + if (rg->to > last_accounted_offset)
> >>> + last_accounted_offset = rg->to;
> >>> + continue;
> >>> + }
> >>>
> >>> - /* Check for and consume any regions we now overlap with. */
> >>> - nrg = rg;
> >>> - list_for_each_entry_safe (rg, trg, rg->link.prev, link) {
> >>> - if (&rg->link == head)
> >>> - break;
> >>> + /* When we find a region that starts beyond our range, we've
> >>> + * finished.
> >>> + */
> >>> if (rg->from > t)
> >>> break;
> >>>
> >>> - /* We overlap with this area, if it extends further than
> >>> - * us then we must extend ourselves. Account for its
> >>> - * existing reservation.
> >>> + /* Add an entry for last_accounted_offset -> rg->from, and
> >>> + * update last_accounted_offset.
> >>> */
> >>> - if (rg->to > t) {
> >>> - chg += rg->to - t;
> >>> - t = rg->to;
> >>> + if (rg->from > last_accounted_offset) {
> >>> + add += rg->from - last_accounted_offset;
> >>> + if (!count_only) {
> >>> + nrg = get_file_region_entry_from_cache(
> >>> + resv, last_accounted_offset, rg->from);
> >>> + list_add(&nrg->link, rg->link.prev);
> >>> + } else if (regions_needed)
> >>> + *regions_needed += 1;
> >>> }
> >>> - chg -= rg->to - rg->from;
> >>>
> >>> - if (!count_only && rg != nrg) {
> >>> - list_del(&rg->link);
> >>> - kfree(rg);
> >>> - }
> >>> + last_accounted_offset = rg->to;
> >>
> >> That last assignment is unneeded. Correct?
> >>
> >
> > Not to make you nervous, but this assignment is needed.
> >
> > The basic idea is that there are 2 loop invariants here:
> > 1. Everything before last_accounted_offset is filled in with file_regions.
> > 2. rg points to the first region past last_account_offset.
> >
> > Each loop iteration compares rg->from to last_accounted_offset, and if
> > there is a gap, it creates a new region to fill this gap. Then this
> > assignment restores loop invariant #2 by assigning
> > last_accounted_offset to rg->to, since now everything before rg->to is
> > filled in with file_regions.
> >
>
> My apologies!
>
> >>> }
> >>>
> >>> - if (!count_only) {
> >>> - nrg->from = f;
> >>> - nrg->to = t;
> >>> + /* Handle the case where our range extends beyond
> >>> + * last_accounted_offset.
> >>> + */
> >>> + if (last_accounted_offset < t) {
> >>> + add += t - last_accounted_offset;
> >>> + if (!count_only) {
> >>> + nrg = get_file_region_entry_from_cache(
> >>> + resv, last_accounted_offset, t);
> >>> + list_add(&nrg->link, rg->link.prev);
> >>> + } else if (regions_needed)
> >>> + *regions_needed += 1;
> >>> + last_accounted_offset = t;
>
> The question about an unnecessary assignment was supposed to be
> directed at the above line.
>
Oh, yes. That assignment is completely unnecessary; the function just
exits after pretty much. Will remove, thanks!
> --
> Mike Kravetz
>
>
> >>> }
> >>>
> >>> - return chg;
> >>> + return add;
> >>> }
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