On Wed, Sep 17, 2025 at 6:38 AM David Hildenbrand <[email protected]> wrote: > > On 15.09.25 18:36, Kalesh Singh wrote: > > The checks against sysctl_max_map_count are open-coded in multiple > > places. While simple checks are manageable, the logic in places like > > mremap.c involves arithmetic with magic numbers that can be difficult > > to reason about. e.g. ... >= sysctl_max_map_count - 3 > > > > To improve readability and centralize the logic, introduce a new helper, > > vma_count_remaining(). This function returns the VMA count headroom > > available for a givine process. > > s/givine/given/ > > s/process/mm/ > > > > > The most common case of checking for a single new VMA can be done with > > the convenience helper has_vma_count_remaining(): > > > > if (!vma_count_remaining(mm)) > > > > And the complex checks in mremap.c become clearer by expressing the > > required capacity directly: > > > > if (vma_count_remaining(mm) < 4) > > > > While a capacity-based function could be misused (e.g., with an > > incorrect '<' vs '<=' comparison), the improved readability at the call > > sites makes such errors less likely than with the previous open-coded > > arithmetic. > > > > As part of this change, sysctl_max_map_count is made static to > > mm/mmap.c to improve encapsulation. > > > > Cc: Andrew Morton <[email protected]> > > Cc: David Hildenbrand <[email protected]> > > Cc: "Liam R. Howlett" <[email protected]> > > Cc: Lorenzo Stoakes <[email protected]> > > Cc: Mike Rapoport <[email protected]> > > Cc: Minchan Kim <[email protected]> > > Cc: Pedro Falcato <[email protected]> > > Signed-off-by: Kalesh Singh <[email protected]> > > --- > > [...] > > > /* > > @@ -1504,6 +1504,25 @@ struct vm_area_struct *_install_special_mapping( > > int sysctl_legacy_va_layout; > > #endif > > > > +static int sysctl_max_map_count __read_mostly = DEFAULT_MAX_MAP_COUNT; > > + > > +/** > > + * vma_count_remaining - Determine available VMA slots > > + * @mm: The memory descriptor for the process. > > + * > > + * Check how many more VMAs can be created for the given @mm > > + * before hitting the sysctl_max_map_count limit. > > + * > > + * Return: The number of new VMAs the process can accommodate. > > + */ > > +int vma_count_remaining(const struct mm_struct *mm) > > +{ > > + const int map_count = mm->map_count; > > + const int max_count = sysctl_max_map_count; > > If we worry about rare races (sysctl_max_map_count changing?) we should > probably force a single read through READ_ONCE()? > > Otherwise one might trick vma_count_remaining() into returning a > negative number I assume.
Agreed, I'll add the READ_ONCE when resending. Thanks, Kalesh > > > + > > + return (max_count > map_count) ? (max_count - map_count) : 0; > > +} > > Nothing else jumped at me. > > Not sure what the buildbot complains about but I'm sure you'll figure it > out :) > > -- > Cheers > > David / dhildenb >
