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Message-ID: <diqz34co8zaw.fsf@ackerleytng-ctop.c.googlers.com>
Date: Wed, 28 May 2025 09:39:35 -0700
From: Ackerley Tng <ackerleytng@...gle.com>
To: Yan Zhao <yan.y.zhao@...el.com>
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Subject: Re: [RFC PATCH v2 39/51] KVM: guest_memfd: Merge and truncate on fallocate(PUNCH_HOLE)
Yan Zhao <yan.y.zhao@...el.com> writes:
> On Wed, May 14, 2025 at 04:42:18PM -0700, Ackerley Tng wrote:
>> Merge and truncate on fallocate(PUNCH_HOLE), but if the file is being
>> closed, defer merging to folio_put() callback.
>>
>> Change-Id: Iae26987756e70c83f3b121edbc0ed0bc105eec0d
>> Signed-off-by: Ackerley Tng <ackerleytng@...gle.com>
>> ---
>> virt/kvm/guest_memfd.c | 76 +++++++++++++++++++++++++++++++++++++-----
>> 1 file changed, 68 insertions(+), 8 deletions(-)
>>
>> diff --git a/virt/kvm/guest_memfd.c b/virt/kvm/guest_memfd.c
>> index cb426c1dfef8..04b1513c2998 100644
>> --- a/virt/kvm/guest_memfd.c
>> +++ b/virt/kvm/guest_memfd.c
>> @@ -859,6 +859,35 @@ static int kvm_gmem_restructure_folios_in_range(struct inode *inode,
>> return ret;
>> }
>>
>> +static long kvm_gmem_merge_truncate_indices(struct inode *inode, pgoff_t index,
>> + size_t nr_pages)
>> +{
>> + struct folio *f;
>> + pgoff_t unused;
>> + long num_freed;
>> +
>> + unmap_mapping_pages(inode->i_mapping, index, nr_pages, false);
>> +
>> + if (!kvm_gmem_has_safe_refcount(inode->i_mapping, index, nr_pages, &unused))
Yan, thank you for your reviews!
> Why is kvm_gmem_has_safe_refcount() checked here, but not in
> kvm_gmem_zero_range() within kvm_gmem_truncate_inode_range() in patch 33?
>
The contract for guest_memfd with HugeTLB pages is that if holes are
punched in any ranges less than a full huge page, no pages are removed
from the filemap. Those ranges are only zeroed.
In kvm_gmem_zero_range(), we never remove any folios, and so there is no
need to merge. If there's no need to merge, then we don't need to check
for a safe refcount, and can just proceed to zero.
kvm_gmem_merge_truncate_indices() is only used during hole punching and
not when the file is closed. Hole punch vs file closure is checked using
mapping_exiting(inode->i_mapping).
During a hole punch, we will only allow truncation if there are no
unexpected refcounts on any subpages, hence this
kvm_gmem_has_safe_refcount() check.
>> + return -EAGAIN;
>> +
>
> Rather than merging the folios, could we simply call kvm_gmem_truncate_indices()
> instead?
>
> num_freed = kvm_gmem_truncate_indices(inode->i_mapping, index, nr_pages);
> return num_freed;
>
We could do this too, but then that would be deferring the huge page
merging to the folio_put() callback and eventually the kernel worker
thread.
My goal here is to try to not to defer merging and freeing as much as
possible so that most of the page/memory operations are
synchronous, because synchronous operations are more predictable.
As an example of improving predictability, in one of the selftests, I do
a hole punch and then try to allocate again. Because the merging and
freeing of the HugeTLB page sometimes takes too long, the allocation
sometimes fails: the guest_memfd's subpool hadn't yet received the freed
page back. With a synchronous truncation, the truncation may take
longer, but the selftest predictably passes.
>> + f = filemap_get_folio(inode->i_mapping, index);
>> + if (IS_ERR(f))
>> + return 0;
>> +
>> + /* Leave just filemap's refcounts on the folio. */
>> + folio_put(f);
>> +
>> + WARN_ON(kvm_gmem_merge_folio_in_filemap(inode, f));
>> +
>> + num_freed = folio_nr_pages(f);
>> + folio_lock(f);
>> + truncate_inode_folio(inode->i_mapping, f);
>> + folio_unlock(f);
>> +
>> + return num_freed;
>> +}
>> +
>> #else
>>
>> static inline int kvm_gmem_try_split_folio_in_filemap(struct inode *inode,
>> @@ -874,6 +903,12 @@ static int kvm_gmem_restructure_folios_in_range(struct inode *inode,
>> return 0;
>> }
>>
>> +static long kvm_gmem_merge_truncate_indices(struct inode *inode, pgoff_t index,
>> + size_t nr_pages)
>> +{
>> + return 0;
>> +}
>> +
>> #endif
>>
>> #else
>> @@ -1182,8 +1217,10 @@ static long kvm_gmem_truncate_indices(struct address_space *mapping,
>> *
>> * Removes folios beginning @index for @nr_pages from filemap in @inode, updates
>> * inode metadata.
>> + *
>> + * Return: 0 on success and negative error otherwise.
>> */
>> -static void kvm_gmem_truncate_inode_aligned_pages(struct inode *inode,
>> +static long kvm_gmem_truncate_inode_aligned_pages(struct inode *inode,
>> pgoff_t index,
>> size_t nr_pages)
>> {
>> @@ -1191,19 +1228,34 @@ static void kvm_gmem_truncate_inode_aligned_pages(struct inode *inode,
>> long num_freed;
>> pgoff_t idx;
>> void *priv;
>> + long ret;
>>
>> priv = kvm_gmem_allocator_private(inode);
>> nr_per_huge_page = kvm_gmem_allocator_ops(inode)->nr_pages_in_folio(priv);
>>
>> + ret = 0;
>> num_freed = 0;
>> for (idx = index; idx < index + nr_pages; idx += nr_per_huge_page) {
>> - num_freed += kvm_gmem_truncate_indices(
>> - inode->i_mapping, idx, nr_per_huge_page);
>> + if (mapping_exiting(inode->i_mapping) ||
>> + !kvm_gmem_has_some_shared(inode, idx, nr_per_huge_page)) {
>> + num_freed += kvm_gmem_truncate_indices(
>> + inode->i_mapping, idx, nr_per_huge_page);
>> + } else {
>> + ret = kvm_gmem_merge_truncate_indices(inode, idx,
>> + nr_per_huge_page);
>> + if (ret < 0)
>> + break;
>> +
>> + num_freed += ret;
>> + ret = 0;
>> + }
>> }
>>
>> spin_lock(&inode->i_lock);
>> inode->i_blocks -= (num_freed << PAGE_SHIFT) / 512;
>> spin_unlock(&inode->i_lock);
>> +
>> + return ret;
>> }
>>
>> /**
>> @@ -1252,8 +1304,10 @@ static void kvm_gmem_zero_range(struct address_space *mapping,
>> *
>> * Removes full (huge)pages from the filemap and zeroing incomplete
>> * (huge)pages. The pages in the range may be split.
>> + *
>> + * Return: 0 on success and negative error otherwise.
>> */
>> -static void kvm_gmem_truncate_inode_range(struct inode *inode, loff_t lstart,
>> +static long kvm_gmem_truncate_inode_range(struct inode *inode, loff_t lstart,
>> loff_t lend)
>> {
>> pgoff_t full_hpage_start;
>> @@ -1263,6 +1317,7 @@ static void kvm_gmem_truncate_inode_range(struct inode *inode, loff_t lstart,
>> pgoff_t start;
>> pgoff_t end;
>> void *priv;
>> + long ret;
>>
>> priv = kvm_gmem_allocator_private(inode);
>> nr_per_huge_page = kvm_gmem_allocator_ops(inode)->nr_pages_in_folio(priv);
>> @@ -1279,10 +1334,11 @@ static void kvm_gmem_truncate_inode_range(struct inode *inode, loff_t lstart,
>> kvm_gmem_zero_range(inode->i_mapping, start, zero_end);
>> }
>>
>> + ret = 0;
>> if (full_hpage_end > full_hpage_start) {
>> nr_pages = full_hpage_end - full_hpage_start;
>> - kvm_gmem_truncate_inode_aligned_pages(inode, full_hpage_start,
>> - nr_pages);
>> + ret = kvm_gmem_truncate_inode_aligned_pages(
>> + inode, full_hpage_start, nr_pages);
>> }
>>
>> if (end > full_hpage_end && end > full_hpage_start) {
>> @@ -1290,6 +1346,8 @@ static void kvm_gmem_truncate_inode_range(struct inode *inode, loff_t lstart,
>>
>> kvm_gmem_zero_range(inode->i_mapping, zero_start, end);
>> }
>> +
>> + return ret;
>> }
>>
>> static long kvm_gmem_punch_hole(struct inode *inode, loff_t offset, loff_t len)
>> @@ -1298,6 +1356,7 @@ static long kvm_gmem_punch_hole(struct inode *inode, loff_t offset, loff_t len)
>> pgoff_t start = offset >> PAGE_SHIFT;
>> pgoff_t end = (offset + len) >> PAGE_SHIFT;
>> struct kvm_gmem *gmem;
>> + long ret;
>>
>> /*
>> * Bindings must be stable across invalidation to ensure the start+end
>> @@ -1308,8 +1367,9 @@ static long kvm_gmem_punch_hole(struct inode *inode, loff_t offset, loff_t len)
>> list_for_each_entry(gmem, gmem_list, entry)
>> kvm_gmem_invalidate_begin_and_zap(gmem, start, end);
>>
>> + ret = 0;
>> if (kvm_gmem_has_custom_allocator(inode)) {
>> - kvm_gmem_truncate_inode_range(inode, offset, offset + len);
>> + ret = kvm_gmem_truncate_inode_range(inode, offset, offset + len);
>> } else {
>> /* Page size is PAGE_SIZE, so use optimized truncation function. */
>> truncate_inode_pages_range(inode->i_mapping, offset, offset + len - 1);
>> @@ -1320,7 +1380,7 @@ static long kvm_gmem_punch_hole(struct inode *inode, loff_t offset, loff_t len)
>>
>> filemap_invalidate_unlock(inode->i_mapping);
>>
>> - return 0;
>> + return ret;
>> }
>>
>> static long kvm_gmem_allocate(struct inode *inode, loff_t offset, loff_t len)
>> --
>> 2.49.0.1045.g170613ef41-goog
>>
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