We have this:
static inline bool blk_queue_may_bounce(struct request_queue *q)
{
return IS_ENABLED(CONFIG_BOUNCE) &&
q->limits.bounce == BLK_BOUNCE_HIGH &&
max_low_pfn >= max_pfn;
}
static inline struct bio *blk_queue_bounce(struct bio *bio,
struct request_queue *q)
{
if (unlikely(blk_queue_may_bounce(q) && bio_has_data(bio)))
return __blk_queue_bounce(bio, q);
return bio;
}
Now, the last term in expression in blk_queue_may_bounce() is
true only on the boxen where max_pfn is no greater than max_low_pfn.
Unless I'm very confused, that's the boxen where we don't *have*
any highmem pages.
What's more, consider this:
static __init int init_emergency_pool(void)
{
int ret;
#ifndef CONFIG_MEMORY_HOTPLUG
if (max_pfn <= max_low_pfn)
return 0;
#endif
ret = mempool_init_page_pool(&page_pool, POOL_SIZE, 0);
BUG_ON(ret);
pr_info("pool size: %d pages\n", POOL_SIZE);
init_bounce_bioset();
return 0;
}
On the same boxen (assuming we've not hotplug) page_pool won't be set up
at all, so we wouldn't be able to bounce any highmem page if we ever
ran into one.
AFAICS, this condition is backwards - it should be
static inline bool blk_queue_may_bounce(struct request_queue *q)
{
return IS_ENABLED(CONFIG_BOUNCE) &&
q->limits.bounce == BLK_BOUNCE_HIGH &&
max_low_pfn < max_pfn;
}
What am I missing here?
