Joao Martins <joao.m.martins@xxxxxxxxxx> writes: > In preparation for device-dax for using hugetlbfs compound page tail > deduplication technique, move the comment block explanation into a > common place in Documentation/vm. > > Cc: Muchun Song <songmuchun@xxxxxxxxxxxxx> > Cc: Mike Kravetz <mike.kravetz@xxxxxxxxxx> > Suggested-by: Dan Williams <dan.j.williams@xxxxxxxxx> > Signed-off-by: Joao Martins <joao.m.martins@xxxxxxxxxx> > Reviewed-by: Muchun Song <songmuchun@xxxxxxxxxxxxx> > Reviewed-by: Dan Williams <dan.j.williams@xxxxxxxxx> > --- > Documentation/vm/index.rst | 1 + > Documentation/vm/vmemmap_dedup.rst | 175 +++++++++++++++++++++++++++++ > mm/hugetlb_vmemmap.c | 168 +-------------------------- > 3 files changed, 177 insertions(+), 167 deletions(-) > create mode 100644 Documentation/vm/vmemmap_dedup.rst Thanks for remembering to add this to the index.rst file! That said, I get the impression you didn't actually build the docs afterward and look at the result; there are a number of things here that won't render the way you might like. > diff --git a/Documentation/vm/index.rst b/Documentation/vm/index.rst > index 44365c4574a3..2fb612bb72c9 100644 > --- a/Documentation/vm/index.rst > +++ b/Documentation/vm/index.rst > @@ -37,5 +37,6 @@ algorithms. If you are looking for advice on simply allocating memory, see the > transhuge > unevictable-lru > vmalloced-kernel-stacks > + vmemmap_dedup > z3fold > zsmalloc > diff --git a/Documentation/vm/vmemmap_dedup.rst b/Documentation/vm/vmemmap_dedup.rst > new file mode 100644 > index 000000000000..8143b2ce414d > --- /dev/null > +++ b/Documentation/vm/vmemmap_dedup.rst > @@ -0,0 +1,175 @@ > +.. SPDX-License-Identifier: GPL-2.0 > + > +.. _vmemmap_dedup: This label isn't needed, I'd take it out. > +================================== > +Free some vmemmap pages of HugeTLB > +================================== > + > +The struct page structures (page structs) are used to describe a physical > +page frame. By default, there is a one-to-one mapping from a page frame to > +it's corresponding page struct. > + > +HugeTLB pages consist of multiple base page size pages and is supported by > +many architectures. See hugetlbpage.rst in the Documentation directory for > +more details. On the x86-64 architecture, HugeTLB pages of size 2MB and 1GB > +are currently supported. Since the base page size on x86 is 4KB, a 2MB > +HugeTLB page consists of 512 base pages and a 1GB HugeTLB page consists of > +4096 base pages. For each base page, there is a corresponding page struct. > + > +Within the HugeTLB subsystem, only the first 4 page structs are used to > +contain unique information about a HugeTLB page. __NR_USED_SUBPAGE provides > +this upper limit. The only 'useful' information in the remaining page structs > +is the compound_head field, and this field is the same for all tail pages. > + > +By removing redundant page structs for HugeTLB pages, memory can be returned > +to the buddy allocator for other uses. > + > +Different architectures support different HugeTLB pages. For example, the > +following table is the HugeTLB page size supported by x86 and arm64 > +architectures. Because arm64 supports 4k, 16k, and 64k base pages and > +supports contiguous entries, so it supports many kinds of sizes of HugeTLB > +page. > + > ++--------------+-----------+-----------------------------------------------+ > +| Architecture | Page Size | HugeTLB Page Size | > ++--------------+-----------+-----------+-----------+-----------+-----------+ > +| x86-64 | 4KB | 2MB | 1GB | | | > ++--------------+-----------+-----------+-----------+-----------+-----------+ > +| | 4KB | 64KB | 2MB | 32MB | 1GB | > +| +-----------+-----------+-----------+-----------+-----------+ > +| arm64 | 16KB | 2MB | 32MB | 1GB | | > +| +-----------+-----------+-----------+-----------+-----------+ > +| | 64KB | 2MB | 512MB | 16GB | | > ++--------------+-----------+-----------+-----------+-----------+-----------+ > + > +When the system boot up, every HugeTLB page has more than one struct page > +structs which size is (unit: pages): > + > + struct_size = HugeTLB_Size / PAGE_SIZE * sizeof(struct page) / PAGE_SIZE This, for example, needs to be in a literal block or you won't get what you expect; that's true of all of the code samples and ascii-art sections. Easiest way to do that is to end the preceding text line with :: instead of : Thanks, jon
