| Message ID | 20201111204008.21332-6-zi.yan@sent.com (mailing list archive) |
|---|---|
| State | New |
| Headers | show |
| Series | Split huge pages to any lower order pages. | expand |
On 11/11/20 12:40 PM, Zi Yan wrote: > From: Zi Yan <ziy@nvidia.com> > > 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 <ziy@nvidia.com> > --- > 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).
On 12 Nov 2020, at 17:08, Ralph Campbell wrote: > On 11/11/20 12:40 PM, Zi Yan wrote: >> From: Zi Yan <ziy@nvidia.com> >> >> 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 <ziy@nvidia.com> >> --- >> 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
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; } /*