On Mon, Mar 30, 2020 at 05:06:49PM -0700, Matthew Wilcox wrote: >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(it is slightly out of date; the XArray actually supports storing unaligned 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? I am not clear how it supports unaligned pointers. So maybe not now. Actually, I still not get the point between page size and valid pointer. Why a valid pointer couldn't be less than 4096? The first page in address space is handled differently? Maybe I miss some point. I'd appreciate it if you'd share some light. Thanks -- Wei Yang Help you, Help me
