tjh.dev / kernel
Linux kernel mirror (for testing) git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git
kernel os linux
1
fork

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mm: Introduce mm_struct.has_pinned

(Commit message majorly collected from Jason Gunthorpe)

Reduce the chance of false positive from page_maybe_dma_pinned() by
keeping track if the mm_struct has ever been used with pin_user_pages().
This allows cases that might drive up the page ref_count to avoid any
penalty from handling dma_pinned pages.

Future work is planned, to provide a more sophisticated solution, likely
to turn it into a real counter. For now, make it atomic_t but use it as
a boolean for simplicity.

Suggested-by: Jason Gunthorpe <jgg@ziepe.ca>
Signed-off-by: Peter Xu <peterx@redhat.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

authored by

Peter Xu and committed by
Linus Torvalds 008cfe44 a1bffa48

+17
+10
include/linux/mm_types.h
··· 436 436 */ 437 437 atomic_t mm_count; 438 438 439 + /** 440 + * @has_pinned: Whether this mm has pinned any pages. This can 441 + * be either replaced in the future by @pinned_vm when it 442 + * becomes stable, or grow into a counter on its own. We're 443 + * aggresive on this bit now - even if the pinned pages were 444 + * unpinned later on, we'll still keep this bit set for the 445 + * lifecycle of this mm just for simplicity. 446 + */ 447 + atomic_t has_pinned; 448 + 439 449 #ifdef CONFIG_MMU 440 450 atomic_long_t pgtables_bytes; /* PTE page table pages */ 441 451 #endif
+1
kernel/fork.c
··· 1011 1011 mm_pgtables_bytes_init(mm); 1012 1012 mm->map_count = 0; 1013 1013 mm->locked_vm = 0; 1014 + atomic_set(&mm->has_pinned, 0); 1014 1015 atomic64_set(&mm->pinned_vm, 0); 1015 1016 memset(&mm->rss_stat, 0, sizeof(mm->rss_stat)); 1016 1017 spin_lock_init(&mm->page_table_lock);
+6
mm/gup.c
··· 1255 1255 BUG_ON(*locked != 1); 1256 1256 } 1257 1257 1258 + if (flags & FOLL_PIN) 1259 + atomic_set(&current->mm->has_pinned, 1); 1260 + 1258 1261 /* 1259 1262 * FOLL_PIN and FOLL_GET are mutually exclusive. Traditional behavior 1260 1263 * is to set FOLL_GET if the caller wants pages[] filled in (but has ··· 2662 2659 FOLL_FORCE | FOLL_PIN | FOLL_GET | 2663 2660 FOLL_FAST_ONLY))) 2664 2661 return -EINVAL; 2662 + 2663 + if (gup_flags & FOLL_PIN) 2664 + atomic_set(&current->mm->has_pinned, 1); 2665 2665 2666 2666 if (!(gup_flags & FOLL_FAST_ONLY)) 2667 2667 might_lock_read(&current->mm->mmap_lock);