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sentry: use read(2) host syscall to perform read on disk-backed MemoryFiles #11163

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sentry: use read(2) host syscall to perform read on disk-backed MemoryFiles #11163

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31 changes: 24 additions & 7 deletions pkg/sentry/fsimpl/tmpfs/regular_file.go
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Original file line number Diff line number Diff line change
Expand Up @@ -655,14 +655,8 @@ func (rw *regularFileReadWriter) ReadToBlocks(dsts safemem.BlockSeq) (uint64, er
mr := memmap.MappableRange{uint64(rw.off), uint64(end)}
switch {
case seg.Ok():
// Get internal mappings.
ims, err := rw.file.inode.fs.mf.MapInternal(seg.FileRangeOf(seg.Range().Intersect(mr)), hostarch.Read)
if err != nil {
return done, err
}
n, err := rw.readFromMF(seg.FileRangeOf(seg.Range().Intersect(mr)), dsts)

// Copy from internal mappings.
n, err := safemem.CopySeq(dsts, ims)
done += n
rw.off += uint64(n)
dsts = dsts.DropFirst64(n)
Expand Down Expand Up @@ -812,6 +806,29 @@ exitLoop:
return done, retErr
}

func (rw *regularFileReadWriter) readFromMF(fr memmap.FileRange, dsts safemem.BlockSeq) (uint64, error) {
if rw.file.inode.fs.mf.IsDiskBacked() {
// Disk-backed files are not prepopulated. The safemem.CopySeq() approach
// used below incurs a lot of page faults without page prepopulation, which
// causes a lot of context switching. Use read(2) host syscall instead,
// which makes one context switch and faults all the pages that are touched
// during the read.
return hostfd.Preadv2(
int32(rw.file.inode.fs.mf.FD()), // fd
dsts.TakeFirst64(fr.Length()), // dsts
int64(fr.Start), // offset
0, // flags
)
}
// Get internal mappings.
ims, err := rw.file.inode.fs.mf.MapInternal(fr, hostarch.Read)
if err != nil {
return 0, err
}
// Copy from internal mappings.
return safemem.CopySeq(dsts, ims)
}

func (rw *regularFileReadWriter) writeToMF(fr memmap.FileRange, srcs safemem.BlockSeq) (uint64, error) {
if rw.file.inode.fs.mf.IsDiskBacked() {
// Disk-backed files are not prepopulated. The safemem.CopySeq() approach
Expand Down