On 15.01.21 11:10, Yafang Shao wrote: > On Fri, Jan 15, 2021 at 4:39 PM David Hildenbrand <david@xxxxxxxxxx> wrote: >> >> On 15.01.21 07:13, Yafang Shao wrote: >>> There're totally __NR_PAGEFLAGS page flags, but the type of page->flags is >>> unsigned long, that makes the value of page->flags a little misleading when >>> it is printed to the user. We'd better print the real pages flags, instead >>> of the whole 64bits including the random values in the useless high bits. >> >> No, these are *not* random values. They include the nid, zid, and >> section_nr - which are helpful to have at hand when debugging, or >> detecting that something might be messed up there. >> > > Thanks for the explanation. I just noticed the page-flags layout in > page-flags-layout.h. > >>> >>> There're two choices to achieve that, one of which is clear the useless >> >> Again, not useless. >> >>> high bits when we initlize the page->flags, the other is don't print the >>> high bits when it is showed to the user. The latter one is better because >>> it is in the slow path and the performance won't be impacted. >>> >>> Before that change, the output is, >>> [ 8846.517809] INFO: Slab 0x00000000f42a2c60 objects=33 used=3 fp=0x0000000060d32ca8 flags=0x17ffffc0010200(slab|head) >>> >>> After that change, the output is, >>> [ 8843.757770] INFO: Slab 0x00000000f0e98335 objects=33 used=3 fp=0x00000000b643c7d8 flags=0x10200(slab|head) >>> >> >> Nack to the current approach. If you're going to strip this information, >> you should expose it differently. E.g., printing page_zonenum() or >> page_to_nid(). But still, then we might lose valuable information of >> bits stored in there that shouldn't have been set. >> > > How about changing the implementation of pGp in printk() ? > In the new implementation of pGp we can dump the full information of > page->flags, rather than the flag's name only. > > For example, > 0xXXXXXXXX(node n, nid n, ..., slab|head) > > That will make it easier to understand, as it is not easy to look into > the detail of page-flags layout. Not completely opposed to this. Just keep in mind that the information stored in the high bits differs per configuration. See include/linux/page-flags-layout.h So to dump reliably, you would have to consider all different flavors, looking at SECTIONS_WIDTH, NODES_WIDTH, ZONES_WIDTH, LAST_CPUPID_WIDTH, KASAN_TAG_WIDTH Might be helpful for other users as well, like in mm/memory-failure.c -- Thanks, David / dhildenb
