On 8/31/2023 12:44 AM, Ackerley Tng wrote:
Binbin Wu [off-list ref] writes:
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<snip>
+static long kvm_gmem_allocate(struct inode *inode, loff_t offset, loff_t len)
+{
+ struct address_space *mapping = inode->i_mapping;
+ pgoff_t start, index, end;
+ int r;
+
+ /* Dedicated guest is immutable by default. */
+ if (offset + len > i_size_read(inode))
+ return -EINVAL;
+
+ filemap_invalidate_lock_shared(mapping);
+
+ start = offset >> PAGE_SHIFT;
+ end = (offset + len) >> PAGE_SHIFT;
+
+ r = 0;
+ for (index = start; index < end; ) {
+ struct folio *folio;
+
+ if (signal_pending(current)) {
+ r = -EINTR;
+ break;
+ }
+
+ folio = kvm_gmem_get_folio(inode, index);
+ if (!folio) {
+ r = -ENOMEM;
+ break;
+ }
+
+ index = folio_next_index(folio);
+
+ folio_unlock(folio);
+ folio_put(folio);
May be a dumb question, why we get the folio and then put it immediately?
Will it make the folio be released back to the page allocator?
I was wondering this too, but it is correct.
In filemap_grab_folio(), the refcount is incremented in three places:
+ When the folio is created in filemap_alloc_folio(), it is given a
refcount of 1 in
filemap_alloc_folio() -> folio_alloc() -> __folio_alloc_node() ->
__folio_alloc() -> __alloc_pages() -> get_page_from_freelist() ->
prep_new_page() -> post_alloc_hook() -> set_page_refcounted()
+ Then, in filemap_add_folio(), the refcount is incremented twice:
+ The first is from the filemap (1 refcount per page if this is a
hugepage):
filemap_add_folio() -> __filemap_add_folio() -> folio_ref_add()
+ The second is a refcount from the lru list
filemap_add_folio() -> folio_add_lru() -> folio_get() ->
folio_ref_inc()
In the other path, if the folio exists in the page cache (filemap), the
refcount is also incremented through
filemap_grab_folio() -> __filemap_get_folio() -> filemap_get_entry()
-> folio_try_get_rcu()
I believe all the branches in kvm_gmem_get_folio() are taking a refcount
on the folio while the kernel does some work on the folio like clearing
the folio in clear_highpage() or getting the next index, and then when
done, the kernel does folio_put().
This pattern is also used in shmem and hugetlb. :)
Thanks for your explanation. It helps a lot.
I'm not sure whose refcount the folio_put() in kvm_gmem_allocate() is
dropping though:
+ The refcount for the filemap depends on whether this is a hugepage or
not, but folio_put() strictly drops a refcount of 1.
+ The refcount for the lru list is just 1, but doesn't the page still
remain in the lru list?
I guess the refcount drop here is the one get on the fresh allocation.
Now the filemap has grabbed the folio, so the lifecycle of the folio now is decided by the filemap/inode?
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+
+ /* 64-bit only, wrapping the index should be impossible. */
+ if (WARN_ON_ONCE(!index))
+ break;
+
+ cond_resched();
+ }
+
+ filemap_invalidate_unlock_shared(mapping);
+
+ return r;
+}
+
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