Re: [RFC PATCH 5/6] mm: truncate: split thp to a non-zero order if possible.
On 12 Nov 2020, at 17:08, Ralph Campbell wrote:
> On 11/11/20 12:40 PM, Zi Yan wrote:
>> From: Zi Yan
>>
>> To minimize the number of pages after a truncation, when truncating a
>> THP, we do not need to split it all the way down to order-0. The THP has
>> at most three parts, the part before offset, the part to be truncated,
>> the part left at the end. Use the non-zero minimum of them to decide
>> what order we split the THP to.
>>
>> Signed-off-by: Zi Yan
>> ---
>> mm/truncate.c | 22 --
>> 1 file changed, 20 insertions(+), 2 deletions(-)
>>
>> diff --git a/mm/truncate.c b/mm/truncate.c
>> index 20bd17538ec2..6d8e3c6115bc 100644
>> --- a/mm/truncate.c
>> +++ b/mm/truncate.c
>> @@ -237,7 +237,7 @@ int truncate_inode_page(struct address_space *mapping,
>> struct page *page)
>> bool truncate_inode_partial_page(struct page *page, loff_t start, loff_t
>> end)
>> {
>> loff_t pos = page_offset(page);
>> -unsigned int offset, length;
>> +unsigned int offset, length, left, min_subpage_size = PAGE_SIZE;
>
> Maybe use "remaining" instead of "left" since I think of the latter as the
> length of the
> left side (offset).
Sure. Will change the name.
>
>> if (pos < start)
>> offset = start - pos;
>> @@ -248,6 +248,7 @@ bool truncate_inode_partial_page(struct page *page,
>> loff_t start, loff_t end)
>> length = length - offset;
>> else
>> length = end + 1 - pos - offset;
>> +left = thp_size(page) - offset - length;
>> wait_on_page_writeback(page);
>> if (length == thp_size(page)) {
>> @@ -267,7 +268,24 @@ bool truncate_inode_partial_page(struct page *page,
>> loff_t start, loff_t end)
>> do_invalidatepage(page, offset, length);
>> if (!PageTransHuge(page))
>> return true;
>> -return split_huge_page(page) == 0;
>> +
>> +/*
>> + * find the non-zero minimum of offset, length, and left and use it to
>> + * decide the new order of the page after split
>> + */
>> +if (offset && left)
>> +min_subpage_size = min_t(unsigned int,
>> + min_t(unsigned int, offset, length),
>> + left);
>> +else if (!offset)
>> +min_subpage_size = min_t(unsigned int, length, left);
>> +else /* !left */
>> +min_subpage_size = min_t(unsigned int, length, offset);
>> +
>> +min_subpage_size = max_t(unsigned int, PAGE_SIZE, min_subpage_size);
>> +
>> +return split_huge_page_to_list_to_order(page, NULL,
>> +ilog2(min_subpage_size/PAGE_SIZE)) == 0;
>> }
>
> What if "min_subpage_size" is 1/2 the THP but offset isn't aligned to 1/2?
> Splitting the page in half wouldn't result in a page that could be freed
> but maybe splitting to 1/4 would (assuming the THP is at least 8x PAGE_SIZE).
Is it possible? The whole THP is divided into three parts, offset, length, and
remaining (renamed from left). If offset is not aligned to 1/2, it is either
greater than 1/2 or smaller than 1/2. If it is the former, length and remaining
will be smaller than 1/2, so min_subpage_size cannot be 1/2. If it is the
latter,
min_subpage_size cannot be 1/2 either. Because min_subpage_size is the smallest
non-zero value of offset, length, and remaining. Let me know if I miss anything.
—
Best Regards,
Yan Zi
signature.asc
Description: OpenPGP digital signature
Re: [RFC PATCH 5/6] mm: truncate: split thp to a non-zero order if possible.
On 11/11/20 12:40 PM, Zi Yan wrote:
From: Zi Yan
To minimize the number of pages after a truncation, when truncating a
THP, we do not need to split it all the way down to order-0. The THP has
at most three parts, the part before offset, the part to be truncated,
the part left at the end. Use the non-zero minimum of them to decide
what order we split the THP to.
Signed-off-by: Zi Yan
---
mm/truncate.c | 22 --
1 file changed, 20 insertions(+), 2 deletions(-)
diff --git a/mm/truncate.c b/mm/truncate.c
index 20bd17538ec2..6d8e3c6115bc 100644
--- a/mm/truncate.c
+++ b/mm/truncate.c
@@ -237,7 +237,7 @@ int truncate_inode_page(struct address_space *mapping,
struct page *page)
bool truncate_inode_partial_page(struct page *page, loff_t start, loff_t end)
{
loff_t pos = page_offset(page);
- unsigned int offset, length;
+ unsigned int offset, length, left, min_subpage_size = PAGE_SIZE;
Maybe use "remaining" instead of "left" since I think of the latter as the
length of the
left side (offset).
if (pos < start)
offset = start - pos;
@@ -248,6 +248,7 @@ bool truncate_inode_partial_page(struct page *page, loff_t
start, loff_t end)
length = length - offset;
else
length = end + 1 - pos - offset;
+ left = thp_size(page) - offset - length;
wait_on_page_writeback(page);
if (length == thp_size(page)) {
@@ -267,7 +268,24 @@ bool truncate_inode_partial_page(struct page *page, loff_t
start, loff_t end)
do_invalidatepage(page, offset, length);
if (!PageTransHuge(page))
return true;
- return split_huge_page(page) == 0;
+
+ /*
+* find the non-zero minimum of offset, length, and left and use it to
+* decide the new order of the page after split
+*/
+ if (offset && left)
+ min_subpage_size = min_t(unsigned int,
+min_t(unsigned int, offset, length),
+left);
+ else if (!offset)
+ min_subpage_size = min_t(unsigned int, length, left);
+ else /* !left */
+ min_subpage_size = min_t(unsigned int, length, offset);
+
+ min_subpage_size = max_t(unsigned int, PAGE_SIZE, min_subpage_size);
+
+ return split_huge_page_to_list_to_order(page, NULL,
+ ilog2(min_subpage_size/PAGE_SIZE)) == 0;
}
What if "min_subpage_size" is 1/2 the THP but offset isn't aligned to 1/2?
Splitting the page in half wouldn't result in a page that could be freed
but maybe splitting to 1/4 would (assuming the THP is at least 8x PAGE_SIZE).
[RFC PATCH 5/6] mm: truncate: split thp to a non-zero order if possible.
From: Zi Yan
To minimize the number of pages after a truncation, when truncating a
THP, we do not need to split it all the way down to order-0. The THP has
at most three parts, the part before offset, the part to be truncated,
the part left at the end. Use the non-zero minimum of them to decide
what order we split the THP to.
Signed-off-by: Zi Yan
---
mm/truncate.c | 22 --
1 file changed, 20 insertions(+), 2 deletions(-)
diff --git a/mm/truncate.c b/mm/truncate.c
index 20bd17538ec2..6d8e3c6115bc 100644
--- a/mm/truncate.c
+++ b/mm/truncate.c
@@ -237,7 +237,7 @@ int truncate_inode_page(struct address_space *mapping,
struct page *page)
bool truncate_inode_partial_page(struct page *page, loff_t start, loff_t end)
{
loff_t pos = page_offset(page);
- unsigned int offset, length;
+ unsigned int offset, length, left, min_subpage_size = PAGE_SIZE;
if (pos < start)
offset = start - pos;
@@ -248,6 +248,7 @@ bool truncate_inode_partial_page(struct page *page, loff_t
start, loff_t end)
length = length - offset;
else
length = end + 1 - pos - offset;
+ left = thp_size(page) - offset - length;
wait_on_page_writeback(page);
if (length == thp_size(page)) {
@@ -267,7 +268,24 @@ bool truncate_inode_partial_page(struct page *page, loff_t
start, loff_t end)
do_invalidatepage(page, offset, length);
if (!PageTransHuge(page))
return true;
- return split_huge_page(page) == 0;
+
+ /*
+* find the non-zero minimum of offset, length, and left and use it to
+* decide the new order of the page after split
+*/
+ if (offset && left)
+ min_subpage_size = min_t(unsigned int,
+min_t(unsigned int, offset, length),
+left);
+ else if (!offset)
+ min_subpage_size = min_t(unsigned int, length, left);
+ else /* !left */
+ min_subpage_size = min_t(unsigned int, length, offset);
+
+ min_subpage_size = max_t(unsigned int, PAGE_SIZE, min_subpage_size);
+
+ return split_huge_page_to_list_to_order(page, NULL,
+ ilog2(min_subpage_size/PAGE_SIZE)) == 0;
}
/*
--
2.28.0
