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Userfaultfd – Create a file descriptor for handling page faults in user space

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Re: Userfaultfd – Create a file descriptor for handling page faults in user space

#11

Post by kragen »

kentonv wrote: post #7
mattgreenrocks wrote: post #4

Possibly dumb question, but if I have some code that tries to blindly jump to, say, address 0x4000, would userfaultfd let me redirect accesses to 0x4000 to an address I've allocated elsewhere?

Sort of, but not exactly. You can't redirect the program to a different address, but you can respond to the fault by mapping memory at address 0x4000 and placing whatever you want in it. Perhaps, you can map the memory as an alias of some other page, so that they refer to the same underlying physical memory. But the program will still see the memory as living at 0x4000, it won't jump to a different address. (Though I…

I think specifically for 0x4000 you can't map anything there with a default Linux kernel configuration. The minimum address where you can map things now is 0x10000. This broke my StoneKnifeForth and Max Bernstein fixed it: https://github.com/tekknolagi/stoneknifecpp/commit/905105b44...

Re: Userfaultfd – Create a file descriptor for handling page faults in user space

#12

Post by kentonv »

kragen wrote: post #11
kentonv wrote: post #7

Earlier quoted context omitted.

Sort of, but not exactly. You can't redirect the program to a different address, but you can respond to the fault by mapping memory at address 0x4000 and placing whatever you want in it. Perhaps, you can map the memory as an alias of some other page, so that they refer to the same underlying physical memory. But the program will still see the memory as living at 0x4000, it won't jump to a different address. (Though I…

I think specifically for 0x4000 you can't map anything there with a default Linux kernel configuration. The minimum address where you can map things now is 0x10000. This broke my StoneKnifeForth and Max Bernstein fixed it: https://github.com/tekknolagi/stoneknifecpp/commit/905105b44...

Whoops, indeed. Guess you'll have to modify the code that made the jump. Ugh.

Re: Userfaultfd – Create a file descriptor for handling page faults in user space

#13

Post by tekknolagi »

kragen wrote: post #11
kentonv wrote: post #7

Earlier quoted context omitted.

Sort of, but not exactly. You can't redirect the program to a different address, but you can respond to the fault by mapping memory at address 0x4000 and placing whatever you want in it. Perhaps, you can map the memory as an alias of some other page, so that they refer to the same underlying physical memory. But the program will still see the memory as living at 0x4000, it won't jump to a different address. (Though I…

I think specifically for 0x4000 you can't map anything there with a default Linux kernel configuration. The minimum address where you can map things now is 0x10000. This broke my StoneKnifeForth and Max Bernstein fixed it: https://github.com/tekknolagi/stoneknifecpp/commit/905105b44...

Technically Tom fixed it :) He's the Linux whisperer. I just submitted the patch

Re: Userfaultfd – Create a file descriptor for handling page faults in user space

#14

Post by zozbot234 »

annonacct38 wrote: post #8

One of my favorite examples of userfaultfd: transparently mapping databases on cloud storage into RAM. https://tech.nextroll.com/blog/data/2016/11/29/traildb-mmap-...

Could something like this enable SSI distributed computing entirely in user space? Is it practical to run some sort of distributed shared memory, relying on user-managed page faults to ensure coherence?

Re: Userfaultfd – Create a file descriptor for handling page faults in user space

#15

Post by kragen »

tekknolagi wrote: post #13
kragen wrote: post #11

Earlier quoted context omitted.

I think specifically for 0x4000 you can't map anything there with a default Linux kernel configuration. The minimum address where you can map things now is 0x10000. This broke my StoneKnifeForth and Max Bernstein fixed it: https://github.com/tekknolagi/stoneknifecpp/commit/905105b44...

Technically Tom fixed it :) He's the Linux whisperer. I just submitted the patch

Oops! Thanks Tom Hebb!

Re: Userfaultfd – Create a file descriptor for handling page faults in user space

#16

Post by tsavola »

10000truths wrote: post #5
mattgreenrocks wrote: post #4

Possibly dumb question, but if I have some code that tries to blindly jump to, say, address 0x4000, would userfaultfd let me redirect accesses to 0x4000 to an address I've allocated elsewhere?

You don't need userfaultfd for that. Just mmap the same file twice, at different addresses: int mem_fd = memfd_create("", 0); ftruncate(fd, 4096); void* mem = mmap(NULL, 4096, PROT_READ | PROT_WRITE, MAP_SHARED, mem_fd, 0); void* alias = mmap((void*) 0x4000, 4096, PROT_READ | PROT_WRITE, MAP_FIXED | MAP_SHARED, mem_fd, 0); close(mem_fd); // alias now points to the same physical memory as mem This is commonly used to…

Do you have a link or can you suggest search terms for an example? I can't imagine that being useful, as you have to wrap around at some point anyway.

Re: Userfaultfd – Create a file descriptor for handling page faults in user space

#17

Post by toast0 »

tsavola wrote: post #16
10000truths wrote: post #5

Earlier quoted context omitted.

You don't need userfaultfd for that. Just mmap the same file twice, at different addresses: int mem_fd = memfd_create("", 0); ftruncate(fd, 4096); void* mem = mmap(NULL, 4096, PROT_READ | PROT_WRITE, MAP_SHARED, mem_fd, 0); void* alias = mmap((void*) 0x4000, 4096, PROT_READ | PROT_WRITE, MAP_FIXED | MAP_SHARED, mem_fd, 0); close(mem_fd); // alias now points to the same physical memory as mem This is commonly used to…

Do you have a link or can you suggest search terms for an example? I can't imagine that being useful, as you have to wrap around at some point anyway.

I could see it being useful to map the same area twice, adjacently; not specifically at 0x4000 though. You'd still wrap pointers at some point, but as long as sizes are limited to fit into your buffer, you could do memcpy and what not without having to check boundaries. If you were mapped at 0x4000 and aliased at 0x5000, you could memcpy something to 0x4FFF -> 0x5003, and not have to do a split write of 0x4fff and then 0x4000 -> 0x4003.

(If your buffer is bigger than a page, this will still work, just use bigger addresses, etc. if your buffer is smaller than a page, you're stuck)

Re: Userfaultfd – Create a file descriptor for handling page faults in user space

#18

Post by mattgreenrocks »

kentonv wrote: post #7
mattgreenrocks wrote: post #4

Possibly dumb question, but if I have some code that tries to blindly jump to, say, address 0x4000, would userfaultfd let me redirect accesses to 0x4000 to an address I've allocated elsewhere?

Sort of, but not exactly. You can't redirect the program to a different address, but you can respond to the fault by mapping memory at address 0x4000 and placing whatever you want in it. Perhaps, you can map the memory as an alias of some other page, so that they refer to the same underlying physical memory. But the program will still see the memory as living at 0x4000, it won't jump to a different address. (Though I…

Thank you for in-depth answer! It was partially motivated by seeing if userfaultfd could work around mmap_min_addr (it can’t) and partially to see if it is possible to use it to use it for things like emulation.
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