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ensure forced abandoned pages can be accessed after free

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daanx 2024-11-25 18:41:57 -08:00
parent 9b7ac9a1a6
commit 7673aa2517
3 changed files with 26 additions and 21 deletions
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3
include/mimalloc/types.h
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@ -416,7 +416,8 @@ typedef struct mi_segment_s {
// segment fields
struct mi_segment_s* next; // must be the first (non-constant) segment field -- see `segment.c:segment_init`
struct mi_segment_s* prev;
bool was_reclaimed; // true if it was reclaimed (used to limit on-free reclamation)
bool was_reclaimed; // true if it was reclaimed (used to limit reclaim-on-free reclamation)
bool dont_free; // can be temporarily true to ensure the segment is not freed
size_t abandoned; // abandoned pages (i.e. the original owning thread stopped) (`abandoned <= used`)
size_t abandoned_visits; // count how often this segment is visited for reclaiming (to force reclaim if it is too long)

6
src/page.c
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@ -411,10 +411,8 @@ void _mi_page_force_abandon(mi_page_t* page) {
// ensure this page is no longer in the heap delayed free list
_mi_heap_delayed_free_all(heap);
// TODO: can we still access the page as it may have been
// freed and the memory decommitted?
// A way around this is to explicitly unlink this page from
// the heap delayed free list.
// We can still access the page meta-info even if it is freed as we ensure
// in `mi_segment_force_abandon` that the segment is not freed (yet)
if (page->capacity == 0) return; // it may have been freed now
// and now unlink it from the page queue and abandon (or free)

18
src/segment.c
View file

@ -652,6 +652,10 @@ static mi_segment_t* mi_segment_alloc(size_t required, mi_page_kind_t page_kind,
static void mi_segment_free(mi_segment_t* segment, bool force, mi_segments_tld_t* tld) {
MI_UNUSED(force);
mi_assert(segment != NULL);
// in `mi_segment_force_abandon` we set this to true to ensure the segment's memory stays valid
if (segment->dont_free) return;
// don't purge as we are freeing now
mi_segment_remove_all_purges(segment, false /* don't force as we are about to free */, tld);
mi_segment_remove_from_free_queue(segment, tld);
@ -1056,19 +1060,21 @@ static mi_segment_t* mi_segment_try_reclaim(mi_heap_t* heap, size_t block_size,
static void mi_segment_force_abandon(mi_segment_t* segment, mi_segments_tld_t* tld)
{
mi_assert_internal(segment->abandoned < segment->used);
mi_assert_internal(!segment->dont_free);
// ensure the segment does not get free'd underneath us (so we can check if a page has been freed in `mi_page_force_abandon`)
segment->dont_free = true;
// for all pages
for (size_t i = 0; i < segment->capacity; i++) {
mi_page_t* page = &segment->pages[i];
if (page->segment_in_use) {
// ensure used count is up to date and collect potential concurrent frees
_mi_page_free_collect(page, false);
{
// abandon the page if it is still in-use (this will free it if possible as well)
// abandon the page if it is still in-use (this will free the page if possible as well (but not our segment))
mi_assert_internal(segment->used > 0);
if (segment->used == segment->abandoned+1) {
// the last page.. abandon and return as the segment will be abandoned after this
// and we should no longer access it.
segment->dont_free = false;
_mi_page_force_abandon(page);
return;
}
@ -1078,10 +1084,10 @@ static void mi_segment_force_abandon(mi_segment_t* segment, mi_segments_tld_t* t
}
}
}
}
segment->dont_free = false;
mi_assert(segment->used == segment->abandoned);
mi_assert(segment->used == 0);
if (segment->used == 0) {
if (segment->used == 0) { // paranoia
// all free now
mi_segment_free(segment, false, tld);
}