On 01/09/23 at 07:12am, Lorenzo Stoakes wrote: > On Mon, Jan 09, 2023 at 12:35:04PM +0800, Baoquan He wrote: > > Sorry for late reply, just come back from vacation. > > Hope you had a great time! :) Thanks. > > > > > Lei + mutt sounds like a good idea. I relied too much on mbsync in the > > past. > > > > Yeah I'm finding it works well, > https://josefbacik.github.io/kernel/2021/10/18/lei-and-b4.html is a handy guide! Very helpful, will try. > > [snip] > > > Maybe let me rephrase:- > > > > > > - We want to read `count` bytes from `addr` into `buf` > > > - We iterate over _used_ blocks, placing the start/end of each block in `rs`, `re` > > > respectively. > > > - If we hit a block whose start address is above the one in which we are interested then:- > > > - Place a zero byte in the buffer > > > - Increment `addr` by 1 byte > > > - Decrement the `count` by 1 byte > > > - Carry on > > > > > > I am seriously confused as to why we do this? Surely we should be checking > > > whether the range [addr, addr + count) overlaps this block at all, and only then > > > copying the relevant region? > > > > I guessed this could be your concern, but not very sure. That > > code block is copied from vread(), and my considerations are: > > 1) We could starting read from any position of kcore file. /proc/kcore > > is a elf file logically, it's allowed to read from anywhere, right? We > > don't have to read the entire file always. So the vmap_block reading is > > not necessarily page aligned. It's very similar with the empty area > > filling in vread(). > > 2) memset() is doing the byte by byte reading. We can > > change code as below. While we don't save the effort very much, and we > > need introduce an extra local variable to store the value of > > (start - end). > > > > diff --git a/mm/vmalloc.c b/mm/vmalloc.c > > index b054081aa66b..dce4a843a9e8 100644 > > --- a/mm/vmalloc.c > > +++ b/mm/vmalloc.c > > @@ -3576,6 +3576,15 @@ static void vmap_ram_vread(char *buf, char *addr, int count, unsigned long flags > > + if (addr < start) { > > + int num = min(count, (start - add)); > > + memset(buf, 0, count); > > + count -= num; > > + if (count == 0) > > + break; > > + buf -= num; > > + addr -= num; > > + } > > /*it could start reading from the middle of used region*/ > > offset = offset_in_page(addr); > > n = ((re - rs + 1) << PAGE_SHIFT) - offset; > > > > The difference with vread() is that uses a while loop rather than an if clause > so operates over the whole region byte-by-byte, your original would only do this > for 1 byte so now things make a lot more sense! Oops, that 'if clause' is a code bug, I finally got your point until now, my dumb head. > > This approach makes sense though I'd put the count == 0 check first and nit > 'add' should be 'addr'. > > I am happy with either this or a while loop instead of an if which it seems is > what the original issue was! OK, I will think again which one is more appropriate. > > > void *memset(void *s, int c, size_t count) > > { > > char *xs = s; > > > > while (count--) > > *xs++ = c; > > return s; > > } > > > > > > > > It's the fact that blocks are at base page granularity but then this condition > > > is at byte granularity that is confusing to me (again it's _very_ possible I am > > > just being dumb here and missing something, just really want to understand this > > > better :) > > > > I like this kind of reviewing with careful checking and deep thinking. > > For above code block, I think it's a very great point. From my point of > > view, I like the memset version better, it's easier to understand. If we > > all agree, we can change it to take memset way. When I made patches, > > several issues related to patches were hovering in my mind at the same > > time, I did not consider this one so deeply. > > > > Thanks :) I have a particular interest in vmalloc so am happy to dive in with > reviews here! > > > > > > > > > > - vm = va->vm; > > > > > > - vaddr = (char *) vm->addr; > > > > > > - if (addr >= vaddr + get_vm_area_size(vm)) > > > > > > + vaddr = (char *) va->va_start; > > > > > > + size = flags ? va_size(va) : get_vm_area_size(vm); > > > > > > > > > > For example here, I feel that this ternary should be reversed and based on > > > > > whether vm is null, unles we expect vm to ever be non-null _and_ flags to be > > > > > set? > > > > > > > > Now only vm_map_ram area sets flags, all other types has vm not null. > > > > Since those temporary state, e.g vm==NULL, flags==0 case has been > > > > filtered out. Is below you suggested? > > > > > > > > size = (!vm&&flags)? va_size(va) : get_vm_area_size(vm); > > > > or > > > > size = (vm&&!flags)? get_vm_area_size(vm):va_size(va); > > > > > > > > > > Sorry I didn't phrase this very well, my point is that the key thing you're > > > relying on here is whether vm exists in order to use it so I simply meant:- > > > > > > size = vm ? get_vm_area_size(vm) : va_size(va); > > > > > > This just makes it really explicit that you need vm to be non-NULL, and you've > > > already done the flags check before so this should suffice. > > > > Sounds reasonable, I will copy above line you pasted. Thanks a lot. Thanks again for careful reviewing and great suggestions and findings.
