On Mon, Mar 30, 2020 at 10:20:13PM +0000, Wei Yang wrote: > On Mon, Mar 30, 2020 at 07:27:08AM -0700, Matthew Wilcox wrote: > >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. > >> > > So you want to say, the 4096 makes sure XArray will not store an address in > first page? If this is the case, I have two suggestions: > > * use PAGE_SIZE would be more verbose? But also incorrect, because it'll be different on different architectures. It's 4096. That's all. > * a node is an internal entry, do we need to compare with xa_mk_internal() > instead? No. 4096 is better because it's a number which is easily expressible in many CPU instruction sets. 4094 is much less likely to be an easy number to encode. > >(it is slightly out of date; the XArray actually supports storing unaligned > >pointers now, but that's not relevant to this discussion) > > Ok, maybe this document need to update. Did you want to send a patch?
