On Mon, Mar 30, 2020 at 02:13:50PM +0000, Wei Yang wrote: > On Mon, Mar 30, 2020 at 06:49:03AM -0700, Matthew Wilcox wrote: > >On Mon, Mar 30, 2020 at 01:45:19PM +0000, Wei Yang wrote: > >> On Mon, Mar 30, 2020 at 05:50:06AM -0700, Matthew Wilcox wrote: > >> >On Mon, Mar 30, 2020 at 12:36:40PM +0000, Wei Yang wrote: > >> >> As the comment mentioned, we reserved several ranges of internal node > >> >> for tree maintenance, 0-62, 256, 257. This means a node bigger than > >> >> XA_ZERO_ENTRY is a normal node. > >> >> > >> >> The checked on XA_ZERO_ENTRY seems to be more meaningful. > >> > > >> >257-1023 are also reserved, they just aren't used yet. XA_ZERO_ENTRY > >> >is not guaranteed to be the largest reserved entry. > >> > >> Then why we choose 4096? > > > >Because 4096 is the smallest page size supported by Linux, so we're > >guaranteed that anything less than 4096 is not a valid pointer. > > I found this in xarray.rst: > > Normal pointers may be stored in the XArray directly. They must be 4-byte > aligned, which is true for any pointer returned from kmalloc() and > alloc_page(). It isn't true for arbitrary user-space pointers, > nor for function pointers. You can store pointers to statically allocated > objects, as long as those objects have an alignment of at least 4. > > So the document here is not correct? Why do you say that? (it is slightly out of date; the XArray actually supports storing unaligned pointers now, but that's not relevant to this discussion)
