On Wed, Jan 15, 2025 at 12:03 PM Andreas Hindborg <a.hindborg@xxxxxxxxxx> wrote: > > "Lorenzo Stoakes" <lorenzo.stoakes@xxxxxxxxxx> writes: > > > On Tue, Jan 14, 2025 at 10:50:01AM +0100, Alice Ryhl wrote: > >> On Mon, Jan 13, 2025 at 3:45 PM Lorenzo Stoakes > >> <lorenzo.stoakes@xxxxxxxxxx> wrote: > >> > > >> > For a series at v11 where there is broad agreement with maintainers within > >> > > >> > the subsystem which it wraps, perhaps the priority should be to try to have > >> > > >> > the series merged unless there is significant technical objection from the > >> > > >> > rust side? > >> > > >> > > >> > > >> >> > >> > > >> >> How about this: > >> > > >> >> > >> > > >> >> This clears the virtual memory map for the range given by `start` and > >> > > >> >> `size`, dropping refcounts to memory held by the mappings in this range. That > >> > > >> >> is, anonymous memory is completely freed, file-backed memory has its > >> > > >> >> reference count on page cache folio's dropped, any dirty data will still > >> > > >> >> be written back to disk as usual. > >> > > >> > > >> > > >> > Sorry I object to this, 'clears the virtual memory map' is really > >> > > >> > vague. What is already there is better. > >> > > >> > >> > > >> Would you like the proposed paragraph if we replaced "virtual memory > >> > > >> map" with "page table mappings", or do you object to the entirety of the > >> > > >> new suggestion? > >> > > > > >> > > > I object to the suggestion in general. The description is fine as it is. > >> > > > >> > > Ok. I'm raising a flag because I had more questions after reading the > >> > > docstring than before. > >> > > >> > Sure and so I think this is valuable information, and indicates it's > >> > probably worthwhile adding a little extra information on mentioning page > >> > tables. > >> > >> Sorry, I'm a bit lost. What would you like me to add? Perhaps there's > >> an existing file in Documentation/ that I can link to? > > > > Sure no problem, I propose expanding: > > > > /// This clears page table mappings for the range at the leaf level, leaving all other page > > /// tables intact, > > /// anonymous memory is completely freed, file-backed memory has its reference count on page > > /// cache folio's dropped, any dirty data will still be written back to disk as usual. > > > > To include information on page tables. I suggest something like: > > > > /// It may seem odd that we clear at the leaf level, this is however a product > > /// of the page table structure used to map physical memory into a virtual > > /// address space - each virtual address actually consists of a bitmap of array > > /// indices into page tables, which form a hierarchical page table level > > /// structure. > > /// > > /// As a result, each page table level maps a multiple of page table levels > > /// below, and thus span ever increasing ranges of pages. At the leaf or PTE > > /// level, we map the actual physical memory. > > /// > > /// It is here where a zap operates, as it the only place we can be certain of > > /// clearing without impacting any other virtual mappings. It is an > > /// implementation detail as to whether the kernel goes further in freeing > > /// unused page tables, but for the purposes of this operation we must only > > /// assume that the leaf level is cleared. > > > > Alice, Andreas - please let me know if this makes sense/is clear or needs > > further clarification. > > Sounds good to me - thanks. > > @Alice - can we add PTE, PTE entry, PMD, PUD to the vocabulary at the > top? Not sure if it should go here in virt.rs or in mm.rs. If you have > no cycles I can try to add it down the road. Sorry, I don't know these concepts well enough to write about them right now. If there's a documentation page I can link to that's great, but otherwise I think it will have to wait. Alice
