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Page allocator: Fixed minor bugs and added commentary

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This commit is contained in:
Maria Matejka 2022-11-03 12:38:57 +01:00
parent 9d03c3f56c
commit 57308fb277
1 changed files with 32 additions and 10 deletions
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42
sysdep/unix/alloc.c
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@ -50,10 +50,10 @@ struct empty_pages {
};
struct free_pages {
list pages;
list empty;
list pages; /* List of (struct free_page) keeping free pages without releasing them (hot) */
list empty; /* List of (struct empty_pages) keeping invalidated pages mapped for us (cold) */
u16 min, max; /* Minimal and maximal number of free pages kept */
uint cnt; /* Number of empty pages */
uint cnt; /* Number of free pages in list */
event cleanup;
};
@ -87,6 +87,7 @@ extern int shutting_down; /* Shutdown requested. */
void *
alloc_page(void)
{
/* If the system page allocator is goofy, we use posix_memalign to get aligned blocks of memory. */
if (use_fake)
{
void *ptr = NULL;
@ -101,27 +102,34 @@ alloc_page(void)
#ifdef HAVE_MMAP
struct free_pages *fps = &global_free_pages;
/* If there is any free page kept hot, we use it. */
if (fps->cnt)
{
struct free_page *fp = SKIP_BACK(struct free_page, n, HEAD(fps->pages));
rem_node(&fp->n);
/* If the hot-free-page cache is getting short, request the cleanup routine to replenish the cache */
if ((--fps->cnt < fps->min) && !shutting_down)
ev_schedule(&fps->cleanup);
bzero(fp, page_size);
return fp;
}
/* If there is any free page kept cold, we use that. */
if (!EMPTY_LIST(fps->empty))
{
struct empty_pages *ep = HEAD(fps->empty);
/* Either the keeper page contains at least one cold page pointer, return that */
if (ep->pos)
return ep->pages[--ep->pos];
/* Or the keeper page has no more cold page pointer, return the keeper page */
rem_node(&ep->n);
return ep;
}
/* And in the worst case, allocate a new page by mmap() */
return alloc_sys_page();
#endif
}
@ -129,6 +137,7 @@ alloc_page(void)
void
free_page(void *ptr)
{
/* If the system page allocator is goofy, we just free the block and care no more. */
if (use_fake)
{
free(ptr);
@ -139,9 +148,11 @@ free_page(void *ptr)
struct free_pages *fps = &global_free_pages;
struct free_page *fp = ptr;
/* Otherwise, we add the free page to the hot-free-page list */
fp->n = (node) {};
add_tail(&fps->pages, &fp->n);
/* And if there are too many hot free pages, we ask for page cleanup */
if ((++fps->cnt > fps->max) && !shutting_down)
ev_schedule(&fps->cleanup);
#endif
@ -151,11 +162,13 @@ free_page(void *ptr)
static void
global_free_pages_cleanup_event(void *data UNUSED)
{
/* Cleanup on shutdown is ignored. All pages may be kept hot, OS will take care. */
if (shutting_down)
return;
struct free_pages *fps = &global_free_pages;
/* Cleanup may get called when hot free page cache is short of pages. Replenishing. */
while (fps->cnt / 2 < fps->min)
{
struct free_page *fp = alloc_sys_page();
@ -164,22 +177,25 @@ global_free_pages_cleanup_event(void *data UNUSED)
fps->cnt++;
}
int limit = CLEANUP_PAGES_BULK;
while (--limit && (fps->cnt > fps->max / 2))
/* Or the hot free page cache is too big. Moving some pages to the cold free page cache. */
for (int limit = CLEANUP_PAGES_BULK; limit && (fps->cnt > fps->max / 2); fps->cnt--, limit--)
{
struct free_page *fp = SKIP_BACK(struct free_page, n, TAIL(fps->pages));
rem_node(&fp->n);
fps->cnt--;
/* Empty pages are stored as pointers. To store them, we need a pointer block. */
struct empty_pages *ep;
if (EMPTY_LIST(fps->empty) || ((ep = HEAD(fps->empty))->pos == EP_POS_MAX))
{
/* There is either no pointer block or the last block is full. We use this block as a pointer block. */
ep = (struct empty_pages *) fp;
*ep = (struct empty_pages) {};
add_head(&fps->empty, &ep->n);
}
else
{
/* We store this block as a pointer into the first free place
* and tell the OS that the underlying memory is trash. */
ep->pages[ep->pos++] = fp;
if (madvise(fp, page_size,
#ifdef CONFIG_MADV_DONTNEED_TO_FREE
@ -192,7 +208,9 @@ global_free_pages_cleanup_event(void *data UNUSED)
}
}
if (!limit)
/* If the hot free page cleanup hit the limit, re-schedule this routine
* to allow for other routines to run. */
if (fps->cnt > fps->max)
ev_schedule(&fps->cleanup);
}
#endif
@ -203,11 +221,15 @@ resource_sys_init(void)
#ifdef HAVE_MMAP
ASSERT_DIE(global_free_pages.cnt == 0);
/* Check what page size the system supports */
if (!(page_size = sysconf(_SC_PAGESIZE)))
die("System page size must be non-zero");
if (u64_popcount(page_size) == 1)
if ((u64_popcount(page_size) == 1) && (page_size >= (1 << 10)) && (page_size <= (1 << 18)))
{
/* We assume that page size has only one bit and is between 1K and 256K (incl.).
* Otherwise, the assumptions in lib/slab.c (sl_head's num_full range) aren't met. */
struct free_pages *fps = &global_free_pages;
init_list(&fps->pages);
@ -217,7 +239,7 @@ resource_sys_init(void)
}
/* Too big or strange page, use the aligned allocator instead */
log(L_WARN "Got strange memory page size (%lu), using the aligned allocator instead", page_size);
log(L_WARN "Got strange memory page size (%ld), using the aligned allocator instead", (s64) page_size);
use_fake = 1;
#endif