On Fri, Jan 15, 2021 at 6:15 PM David Hildenbrand <david@xxxxxxxxxx> wrote: > > 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 for the suggestion. I will think about it. -- Thanks Yafang
