static unsigned long do_try_to_free_pages(struct zone **zones, gfp_t gfp_mask,
struct scan_control *sc)
/* Take a nap, wait for some writeback to complete */
if (sc->nr_scanned && priority < DEF_PRIORITY - 2)
congestion_wait(WRITE, HZ/10);
/* top priority shrink_caches still had more to do? don't OOM, then */
if (!sc->all_unreclaimable && scan_global_lru(sc))
ret = 1;

I think we change shrink_caches commet with shrink_zone.

And I can't understand that's comment.

What's role sc->all_unreclaimable ?
What benefit do we can get with that code ?
If we don't have that code, What's problem happen ?

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