On 2019/11/23 6:13, Matthew Wilcox wrote: > On Fri, Nov 22, 2019 at 09:23:30AM +0800, zhengbin (A) wrote: >> On 2019/11/22 3:53, Hugh Dickins wrote: >>> On Thu, 21 Nov 2019, zhengbin (A) wrote: >>>> On 2019/11/21 12:52, Hugh Dickins wrote: >>>>> Just a rushed FYI without looking at your patch or comments. >>>>> >>>>> Internally (in Google) we do rely on good tmpfs inode numbers more >>>>> than on those of other get_next_ino() filesystems, and carry a patch >>>>> to mm/shmem.c for it to use 64-bit inode numbers (and separate inode >>>>> number space for each superblock) - essentially, >>>>> >>>>> ino = sbinfo->next_ino++; >>>>> /* Avoid 0 in the low 32 bits: might appear deleted */ >>>>> if (unlikely((unsigned int)ino == 0)) >>>>> ino = sbinfo->next_ino++; >>>>> >>>>> Which I think would be faster, and need less memory, than IDA. >>>>> But whether that is of general interest, or of interest to you, >>>>> depends upon how prevalent 32-bit executables built without >>>>> __FILE_OFFSET_BITS=64 still are these days. >>>> So how google think about this? inode number > 32-bit, but 32-bit executables >>>> cat not handle this? >>> Google is free to limit what executables are run on its machines, >>> and how they are built, so little problem here. >>> >>> A general-purpose 32-bit Linux distribution does not have that freedom, >>> does not want to limit what the user runs. But I thought that by now >>> they (and all serious users of 32-bit systems) were building their own >>> executables with _FILE_OFFSET_BITS=64 (I was too generous with the >>> underscores yesterday); and I thought that defined __USE_FILE_OFFSET64, >>> and that typedef'd ino_t to be __ino64_t. And the 32-bit kernel would >>> have __ARCH_WANT_STAT64, which delivers st_ino as unsigned long long. >>> >>> So I thought that a modern, professional 32-bit executable would be >>> dealing in 64-bit inode numbers anyway. But I am not a system builder, >>> so perhaps I'm being naive. And of course some users may have to support >>> some old userspace, or apps that assign inode numbers to "int" or "long" >>> or whatever. I have no insight into the extent of that problem. >> So how to solve this problem? >> >> 1. tmpfs use ida or other data structure >> >> 2. tmpfs use 64-bit, each superblock a inode number space >> >> 3. do not do anything, If somebody hits this bug, let them solve for themselves >> >> 4. (last_ino change to 64-bit)get_next_ino -->other filesystems will be ok, but it was rejected before > 5. Extend the sbitmap API to allow for growing the bitmap. I had a > look at doing that, and it looks hard. There are a lot of things which > are set up at initialisation and changing them mid-use seems tricky. > Ccing Jens in case he has an opinion. > > 6. Creating a percpu IDA. This doesn't seem too hard. We need a percpu > pointer to an IDA leaf (128 bytes), and a percpu integer which is the > current base for this CPU. At allocation time, find and set the first > free bit in the leaf, and add on the current base. > > If the percpu leaf is full, set the XA_MARK_1 bit on the entry in > the XArray. Then look for any leaves which have both the XA_MARK_0 > and XA_MARK_1 bits set; if there is one, claim it by clearing the > XA_MARK_1 bit. If not, kzalloc a new one and find a free spot for it > in the underlying XArray. > > Freeing an ID is simply ida_free(). That will involve changing the > users of get_next_ino() to call put_ino(), or something. > > This should generally result in similar contention between threads as > the current scheme -- accessing a shared resource every 1024 allocations. > Maybe more often as we try to avoid leaving gaps in the data structure, > or maybe less as we reuse IDs. > > (I've tried to explain what I want here, but appreciate it may be > inscrutable. I can try to explain more, or maybe I should just write > the code myself) Hi willy, do you write this? > > . >
